server+util: Limit max request sizes, prealloc request buffers (#6868)

This commit introduces a few major changes:
 - (Breaking change) Limits now exist for maximum request body sizes.
 - Buffers are preallocated for reading request bodies.
 - Buffers are preallocated for decompressing request bodies.
 - Gzip decoder instances are reused in a `sync.Pool` across requests.

The effect on garbage collection is dramatically fewer GC pauses, giving
a roughly 9% RPS improvement in load tests with gzipped request bodies.
For larger request sizes, the number of GC pauses is dramatically
reduced, although the peak pause time may increase by a few percent.

Implementation notes:
 - The DecodingLimits handler enforces the max request body size both
   through a Content-Length check, and a MaxBytesReader wrapper around
   the payload.
 - The DecodingLimits handler passes the gzip payload size limit down
   using a context key.

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>