Extract shared threading (mutex/cond) wrappers out of pub/sub into a dedicated module

[prateek-kumar-improving]

Summary

The cross-platform threading primitives — the mutex_* and cond_* wrapper functions (pthread on POSIX, CRITICAL_SECTION / CONDITION_VARIABLE on Windows) — currently live in valkey_glide_pubsub_common.{c,h}, because pub/sub introduced them first.

They are now also used by the MONITOR client (valkey_glide_monitor_common.*), which #includes valkey_glide_pubsub_common.h solely to obtain a mutex/condition variable. This is an awkward coupling: MONITOR has nothing to do with pub/sub, yet it depends on the pub/sub module for generic threading utilities.

Proposal

Extract the threading wrappers into a dedicated, neutral module, e.g. valkey_glide_threading.{c,h}:

• Move mutex_init, mutex_lock, mutex_unlock, mutex_destroy
• Move cond_init, cond_wait, cond_timedwait, cond_signal, cond_destroy
• Update valkey_glide_pubsub_common.* and valkey_glide_monitor_common.* to include the new header
• Add the new source file to config.m4, Makefile.frag, and package.xml

Context

Raised in review of #309 (MONITOR command). Deferred from that PR to keep its scope focused; the reviewer (@currantw) agreed to track it separately.

#309 (comment)