[Feature] memory_stat: per-worker memory dump collector

[Feature] memory_stat: per-worker memory dump collector

Introduce src/libserver/memory_stat.{cxx,h} that gathers a UCL dump for
a worker process: OS-level RSS/VmSize breakdown, mempool aggregate plus
per-callsite suggestions, Lua heap usage, and (when WITH_JEMALLOC is
defined) jemalloc mallctl counters and the textual malloc_stats_print
dump. The document is serialized to a tempfile and the descriptor is
passed back over the control pipe with SCM_RIGHTS, mirroring the
existing fuzzy_stat pattern.

Wire the collector into rspamd_control_default_cmd_handler so any
worker registered with the default control handlers transparently
answers RSPAMD_CONTROL_MEMORY_STAT without per-worker boilerplate.