Add server-side statement timeouts: query_timeout (connection) and timeout= (per execute())

Client-side timeouts don't work for async: socket.settimeout() is a no-op on the non-blocking socket, and asyncio.wait_for() only cancels the client coroutine while the server keeps executing the statement (and leaves the connection desynced). So socket_timeout/read_timeout/write_timeout aren't meaningful for async.
Enforce timeouts on the server instead, at two levels:

1. Connection-level query_timeout= applied once after connect as the default for every statement on the connection:

MariaDB:

SET SESSION max_statement_time = <query_timeout>

(seconds)
MySQL:

SET SESSION max_execution_time = <query_timeout × 1000>

(ms)

2. Per-statement timeout= on execute()/executemany(), overrides query_timeout for a single statement:

MariaDB: prefix the statement with

SET STATEMENT max_statement_time=<timeout> FOR <statement>

: one round-trip, works for any statement type.
MySQL: no usable generic per-statement mechanism (MAX_EXECUTION_TIME is a SELECT-only optimizer hint that requires rewriting the query), so per-statement timeout is not supported on MySQL.
The server aborts overrunning statements itself with a clean error, so the connection stays usable. Works for both sync and async.

units differ: MariaDB seconds, MySQL ms

MySQL max_execution_time covers read-only SELECT only, so on MySQL the timeout is effectively SELECT-only and the per-statement override is unavailable.