Performance schema likely/unlikely assume that performance schema
is enabled by default, which causes a performance degradation for
default installations that doesn't have performance schema enabled.

Fixed by changing the likely/unlikely in PS to assume it's
not enabled. This can be changed by compiling with
-DPSI_ON_BY_DEFAULT

Other changes:
- Added psi_likely/psi_unlikely that is depending on
  PSI_ON_BY_DEFAULT. psi_likely() is assumed to be true
  if PS is enabled.
- Added likely/unlikely to some PS interface code.
- Moved pfs_enabled to mysys (was initialized but not used before)
- Added "if (pfs_likely(pfs_enabled))" around calls to PS to avoid
  an extra call if PS is not enabled.
- Moved checking flag_global_instrumention before other flags
  to speed up the case when PS is not enabled.