Pool should spawn a task when connecting in `acquire()`

[abonander]

This way, if the acquire() call is cancelled or times out, the connection can still go through.

The connection task should have its own separate timeout, possibly surfaced as PoolOptions::connect_timeout().

This will likely fix:

• #3315
• #3132
• #2848 (https://github.com/launchbadge/sqlx/issues/2848#issuecomment-1796790935)