Add pool_timeout parameter to prevent pool operation hangs (#431)

[ermakov-oleg]

Summary

Adds pool_timeout parameter to Pool and create_pool() to prevent indefinite hangs in pool operations.

Problem

Pool operations could hang indefinitely, particularly during connection cleanup and cancellation (issue #431).

Solution

pool = await asyncpg.create_pool(dsn, pool_timeout=5.0)

# Now safe to call without explicit timeouts
result = await pool.fetch("SELECT 1")
conn = await pool.acquire()  # Uses pool_timeout as fallback

Changes

• Added pool_timeout parameter with validation (must be > 0 or None)
• Fallback logic: explicit timeout → pool_timeout → infinite wait
• Pool convenience methods (fetch, execute, etc.) now respect pool_timeout
• Full backward compatibility maintained