say: prepare for flightrec dynamic settings

Function say_set_log_callback is not convenient for dynamic
configuration because we need to set callback only once per box run and
we can change flightrec log level many times. Let's make log callback
just a global variable.

Also give related items a bit more comprehensible names (yet not
generic).

Part of https://github.com/tarantool/tarantool-ee/issues/200

NO_DOC=refactoring
NO_TEST=refactoring
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