Micro optimizations

In MariaDB there seem to be no major (>5%) bottlenecks for CPU bound workloads. However there's a whole bunch of more or less easily fixable minor things that waste inexcusable amount of time. Among those:

• one-line virtual methods that can't be inlined
• simple functions defined in source file that can't be inlined
• abuse of likely()/unlikely() (likely in InnoDB, PFS)
• added (sometimes complex) conditions (PFS, WSREP, PROFILING)
• frequent calls of pthread_getspecific() (InnoDB, parser, optimizer)

See linked issues for details.