Fix C23 strchr const qualifier warnings

Fixes #93.

C23's type-generic strchr returns a const char * when passed the const char * command-parameter view, which exposes three qualifier mismatches in cmd_log, cmd_sets, and cmd_sindex under recent libc headers.

• Keep cmd_log's existing in-place separator split explicit with a cast; the command handler already writes a NUL at that separator.
• Mark the cmd_sets and cmd_sindex search pointers const; the latter's stats lookup API now also declares its read-only name parameter const in the header and definition.

Validation: a task-local Zig C23 compiler test using a synthetic C23-style strchr declaration failed with the original declarations (-Werror) and compiled/ran with the corrected pointer types; git diff --check passed. The Aerospike server was not built or started. No real storage/device/cluster/network/user data was used.