Allow modifications of empty profiles

Allow modifications of empty profiles

Add the notion of a memory-only prf_data_t object, indicated by an
empty filespec field and appropriate flags (do not reload, always
dirty, not part of shared trees).  Do nothing when flushing a
memory-only data object to its backing file.  When setting up an empty
profile for read/write access, create a memory-only data object
instead of crashing.

Move prf_data_t mutex initialization into profile_make_prf_data(),
simplifying its callers.

ticket: 9110