Enforce stricter slave lag limits for bot API requests

Enforce stricter slave lag limits for bot API requests

* All users with the bot right sending requests to
  write APIs will get a read-only error when slave
  lag is high. The lag tolerance is configured via
  $wgAPIMaxLagThreshold.
* Making this lower than 'max lag' in the LBFactory
  config means bots get read-only errors before other
  users, which is sensible.
* The default LoadBalancer 'max lag' is now 10 instead
  of 30, which now matches the timeout used for waiting
  on MASTER_POS_WAIT() for ChronologyProtector. This is
  better for assuring that users see their own changes.

Bug: T95501
Change-Id: I56a7f3538278378c67fa3ef5ff0ba4aa6a9a8c64