Avoid duplicate code for parsing dates in different backends
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.
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