SQLite, MySQL and PostgreSQL backends used the same but slightly
different code for parsing the contents of a buffer into std::tm struct.

Replace all these different versions with a single version, copied from
the PostgreSQL variant, which seems like the most complete, in the
common code.

Update MySQL-specific test to not expect parsing a string containing
time without date to yield year 2000, this didn't make any sense at all
and PostgreSQL default of year 1900 makes at least slightly more sense.