client/visitor: deduplicate visitor connection handshake and wrapping (#5219)

Extract two shared helpers to eliminate duplicated code across STCP,
SUDP, and XTCP visitors:

- dialRawVisitorConn: handles ConnectServer + NewVisitorConn handshake
  (auth, sign key, 10s read deadline, error check)
- wrapVisitorConn: handles encryption + pooled compression wrapping,
  returning a recycleFn for pool resource cleanup

SUDP is upgraded from WithCompression to WithCompressionFromPool,
aligning with the pooled compression used by STCP and XTCP.