Change internal plan ID type from uint64 to int64

Change internal plan ID type from uint64 to int64

uint64 was chosen to be consistent with the type used by the query ID,
but the conclusion of a recent discussion for the query ID is that int64
is a better fit as the signed form is shown to the user, for PGSS or
EXPLAIN outputs.

This commit changes the plan ID to use int64, following c3eda50b0648
that has done the same for the query ID.

The plan ID is new to v18, introduced in 2a0cd38da5cc.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/aCvzJNwetyEI3Sgo@paquier.xyz