Reconsider the used method of thread-safety once updating to c-ares 1.23.0+

c-ares added opt-out thread-safety (simple locks around all channel-related functions) in 1.23.0: https://github.com/c-ares/c-ares/pull/636

We're still using an older version, but once we update, we should either leave the c-ares provided lock in, and remove our outer lock in AresChannel.swift, or the opposite: disable c-ares locking, and keep ours. We want to avoid paying the cost for both.

Start with AresChannel.swift and review c-ares 1.23.0's opt-out thread-safety change in PR 636. Compare retaining c-ares locking with removing the outer lock, or disabling c-ares locking while retaining the outer lock; done means selecting one mechanism and avoiding the cost of both.

Written by the indexing model from the issue text.