log: call log_write_flightrec regardless of the log level

Before commit 24323448 ("log: add log.new() function that creates a
new logger"), log_write_flightrec() was called from log_vsay() regardless
of the log level. After, the log level is checked in Lua, so log_vsay()
may not be called. This patch restores the original behaviour by moving
log_write_flightrec() calls to say_default() and say().

Part of https://github.com/tarantool/tarantool-ee/issues/320

NO_DOC=bugfix
NO_CHANGELOG=unreleased
NO_TEST=will be enabled in EE, because there are no flightrec in CE