perf: replace calendar.timegm(utctimetuple()) with integer arithmetic in DateType.serialize

Eliminate the intermediate struct_time allocation in DateType.serialize()
and Encoder.cql_encode_datetime() by using direct timedelta arithmetic
instead of calendar.timegm(v.utctimetuple()).

The old code allocated a 9-field time.struct_time named tuple via
utctimetuple(), then calendar.timegm() disassembled it back to an epoch
integer.  The new code subtracts the epoch datetime directly to get a
timedelta, then extracts days/seconds/microseconds as integers — zero
intermediate object allocations.

Handles both naive (treated as UTC) and timezone-aware datetimes.

DateType.serialize datetime:  1022 -> 232 ns/call  (4.4x faster)
DateType.serialize date:      1369 -> 471 ns/call  (2.9x faster)
BoundStatement.bind (5-col):  4027 -> 3073 ns/call (1.3x faster)

Author: mykaul