ThreadPool: make high priority tickets enabled by default in PS 5.5

Benchmarks show that the high priority tickets idea implemented
previously
(https://blueprints.launchpad.net/percona-server/+spec/priority-scheduling-for-threadpool)
is universal enough to be enabled by default.

That is, the default thread pool behavior should be to always put events
from already started transactions into the high priority queue, as we
believe that results in better performance in vast majority of
cases.

One way to achieve that and keep the current semantics for
--thread-pool-high-prio-tickets at the same time is to make the maximum
possible value (UINT_MAX) a new default instead of 0. This way the old
default behavior can be restored by setting the variable to 0, which
disables the high priority queue.