Metadata manager pr5 (#1930)

# Metadata Manager PR5 Summary

This is the tip of the stack, so the change is described against main. 
Replaces the agent's inline metadata-exchange path with an
internal, agent-owned nixlMDManager that owns a worker thread and routes
each
call to a pluggable, self-contained backend (P2P, ETCD, or TCPStore).
The public
nixlAgent metadata API is unchanged: callers still use the same
sendLocalMD,
sendLocalPartialMD, fetchRemoteMD, and invalidateLocalMD calls with
nixl_opt_args_t.

On main, P2P and ETCD metadata exchange is implemented inline in
nixlAgent on top
of a shared comm-worker thread, with the socket and etcd helpers living
in
nixl_listener.cpp, and there is no TCPStore transport. This change
factors all of
that behavior behind a nixlMetadataBackend contract, moves the worker
thread into
the manager, deletes nixl_listener.cpp, and adds a TCPStore backend.
Observable
behavior of the existing P2P and ETCD paths is preserved.

## What?

Adds the internal manager/backend layer for metadata exchange and three
self-contained backends (P2P, ETCD, TCPStore), and removes the inline
metadata
path and the shared comm worker. The manager is owned by nixlAgentData
and is not
part of the installed public API. TCPStore is a new centralized-store
option
alongside ETCD.

## Why?

On main the metadata transport is entangled with the agent and a shared
comm
thread, so each transport's details live inside nixlAgent and
nixl_listener.cpp.
This creates a clean boundary: the manager owns scheduling and
threading, each
backend owns its transport and I/O, and adding a new backend is one
class behind
the contract with no agent-side or public-API changes.

## How?

Manager - nixlMDManager is owned by nixlAgentData and is the single path
for all
metadata exchange (the env opt-in and the inline fallback are gone). It
owns one
worker thread and routes each call by precedence: a peer address selects
P2P,
otherwise the configured name-addressed store backend.

Backends - nixlMetadataBackend defines the transport boundary. Three
implementations: P2P owns its sockets and listener, ETCD owns its
nixlEtcdClient,
and TCPStore owns an in-house c10d client (no libtorch). Each backend is
self-contained and runs its blocking I/O on the worker; serviceEvents()
drives
the watch / accept / read loops for backends that need them.

Contract - the prepare* methods run on the caller thread: they validate
and
serialize, return a synchronous status, and hand back the transport work
as a
nixlWorkerTask. The task and serviceEvents() run on the manager's worker
thread.
needsWorker() gates whether the worker runs for a given backend.

Cutover - nixl_listener.cpp (the comm-worker loop plus the socket and
etcd
helpers) is deleted. Its loop moves into the manager's worker, the
socket helpers
move into the P2P backend, and the nixlEtcdClient moves into the ETCD
backend.
nixlAgentData drops its comm-thread members and instead implements
nixlMetadataContext (getLocalMD / getLocalPartialMD / loadRemoteMD /
getName /
getConfig / invalidateRemoteMD / submitTask) for the backends.

Scope - No public API change. Existing P2P and ETCD behavior is
preserved, and
all transport I/O now runs on the manager's worker instead of the agent
comm
thread. TCPStore is new and behaves like the other centralized store:
fetchRemoteMD
returns immediately and the caller polls checkRemoteMD, so fetch
semantics are
uniform across P2P, ETCD, and TCPStore.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added a centralized agent metadata manager that unifies peer-to-peer
metadata exchange with centralized backends (ETCD and TCPStore).
* Supports full and partial local metadata publishing, remote metadata
fetch, and remote metadata invalidation.
* **Bug Fixes**
* Reworked metadata routing and lifecycle to provide more consistent
startup/shutdown behavior and reliable remote invalidation/removal
semantics.
* **Tests**
* Added/expanded GTest coverage for manager routing, P2P visibility, and
ETCD/TCPStore publish/fetch/invalidate behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: Asaf Schwartz <aschwartz@nvidia.com>
Co-authored-by: Colin Hirsch <chirsch@nvidia.com>