perf: replace COMMANDS struct format strings with precompiled packers.

Each entry in the COMMANDS table now carries a precompiled
struct.Struct for the fixed-size prefix of its wire format
(HEADER_STRUCT plus any leading "extras" bytes); variable-length
tails (key, value, auth payloads) are concatenated as bytes after
packer.pack(...).

Previously each call site built a fresh format string from
HEADER_STRUCT + the per-command 'struct' suffix and substituted in
the per-call lengths via "%".  This costs a string concat plus a %
format on every call, and -- because the resulting format string
embeds the per-call lengths ('17s', '23s', ...) -- defeats the LRU
cache that the struct module keeps over compiled formats.  Once the
working set of distinct lengths exceeds that cache (100 entries by
default in CPython), every call recompiles its format from scratch.
The hot build loops in get_multi, set_multi, and set_multi_cas paid
this on every key; additionally bind packer.pack and the relevant
COMMANDS / MAGIC / STATUS lookups to locals.

Microbench (500-key get_multi request build, no network): 1181us
-> 441us, ~2.7x.

Author: alexmv