Move the "instr_time" typedef and associated macros into a new header

Move the "instr_time" typedef and associated macros into a new header
file portability/instr_time.h, and add a couple more macros to eliminate
some abstraction leakage we formerly had.  Also update psql to use this
header instead of its own copy of nearly the same code.

This commit in itself is just code cleanup and shouldn't change anything.
It lays some groundwork for the upcoming function-stats patch, though.