Introduce statement_ versions

Add three new functions:

    statement_sysdatetime
    statement_sysdatetimeoffset
    statement_sysutcdatetime

with the same semantics as the base versions, but stable across
multiple extensions within a statement.

An ExecutorStart_hook is used to clear the stored timestamp, and it
is then set on first use of any of these functions, so the cost of
this support to code that does not use it is negligible.