Make database classes handle hyphens in $wgDBname

Make database classes handle hyphens in $wgDBname

* Add DatabaseDomain class to handle passing domains around.
It also can be cast to and from strings, which are of the same
format as wfWikiId() except with hyphens escaped.
* Make IDatabase::getDomainID() use these IDs so they can be
passed into LoadBalancer::getConnection() and friends without
breaking on sites with a hyphen in the DB name.
* Add more LBFactory unit tests for domains.

Bug: T145840
Change-Id: Icfed62b251af8cef706a899197c3ccdb730ef4d1