LoggerFactory: Only check for Psr\Log\LoggerInterface once

LoggerFactory: Only check for Psr\Log\LoggerInterface once

LoggerFactory::getInstance() will be called many times during the course
of handling a typical MediaWiki request. The interface_exists() guard
condition it uses is an attempt to provide an informative error message
when Composer managed libraries are not installed. This check is only
needed on the first invocation of getInstance() to be effective. Using
an additional boolean to guard the interface_exists() call will allow
the PHP runtime to avoid a potentially expensive (at least compared to
a static boolean comparison) function call.

This is the sort of thing that smells of premature optimization, but its
addition is in fact informed by examination of performance reports from
the Wikimedia production environment.

Bug: T115729
Change-Id: I437bcb5326b06145081f2b86f6c4d0c8dc1a318c