Add support for DSL.epoch in Firebird

[lukaseder]

We can calculate the EPOCH of a timestamp in Firebird, e.g. using:

SELECT DATEDIFF(second, TIMESTAMP'1970-01-01 00:00', CURRENT_TIMESTAMP) 
FROM RDB$DATABASE

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See: https://stackoverflow.com/a/27858817/521799

[lukaseder]

We first need a TIMESTAMPDIFF(DatePart, Field, Field) method (see #7900)