Relative Thread Scheduling Priorities for XtraDB

Make priority threads (as defined by [1]) get more CPU time to run. This complements [2] and [3] where priority threads are the first ones to wake up in sync waits.

The implementation is Linux-specific: make use of the fact that setpriority(2) is a per-thread call. A portable alternative would be pthread_setschedparam(), but it's impossible to change the relative thread priority there without changing the thread scheduling policy too, which is a too invasive change.

On Linux, provide an user-exposed tuning option innodb_sched_priority_cleaner=0..39, that corresponds to nice values on Linux 19..-20. If needed, later a mapping from such values to other platform priorities (ie. THREAD_PRIORITY_ABOVE_NORMAL etc. on Windows) might be provided.

Provide similar tuning options for other threads, hidden under UNIV_PERF_DEBUG: innodb_sched_priority_purge, innodb_sched_priority_io, innodb_sched_priority_master.

[1] https://blueprints.launchpad.net/percona-server/+spec/xtradb-thread-priority-flag
[2] https://blueprints.launchpad.net/percona-server/+spec/xtradb-priority-mutex
[3] https://blueprints.launchpad.net/percona-server/+spec/xtradb-priority-rwlock