std::gmtime() has a sad property of using a global static buffer
for returning its value. This is not thread-safe, so its usage
is replaced with gmtime_r, which can accept a local buffer.
While no regressions where observed in this particular area of code,
a similar bug caused failures in alternator, so it's better to simply
replace all std::gmtime calls with their thread-safe counterpart.
Message-Id: <39e91c74de95f8313e6bb0b12114bf12c0e79519.1588589151.git.sarna@scylladb.com>
This change fixes the dependencies between the clock implementation headers. All
the clocks share the common clock offset, but are otherwise independent (though
the `db_clock` does depend on `gc_clock` for time point conversions).