MINOR: log: add log-profile parsing logic

MINOR: log: add log-profile parsing logic

This patch implements prerequisite log-profile struct and parser logic.
It has no effect during runtime for now.

Logformat expressions provided in log-profile "steps" are postchecked
during postparsing for each proxy "log" directive that makes use of a
given profile. (this allows to ensure that the logformat expressions
used in the profile are compatible with proxy using them)