localrepo: give a sigh of relief when getting lock after waiting for it

A message like this was sometimes shown when pushing:
  remote: waiting for lock on repository foo held by 'mercurial:20858'
That could scare users, making them wonder whether the push actually succeeded.

To mitigate that fear, issue an additional "warning" such as:
  got lock after 2 seconds

The return value from lock.lock.lock() was unused - instead we return the
delay.

This also adds the first test coverage for waiting for locks.
discovery: cleanup of variable names and comments