pgsql: Rename RBTXN_PREPARE to RBTXN_IS_PREPARE for better clarificatio

Rename RBTXN_PREPARE to RBTXN_IS_PREPARE for better clarification.

RBTXN_PREPARE flag and rbtxn_prepared macro could be misinterpreted as
either indicating the transaction type (e.g. a prepared transaction or
a normal transaction) or its currentstate (e.g. skipped or its prepare
message is sent), especially after commit 072ee847ad4 introduced the
RBTXN_SENT_PREPARE flag and the rbtxn_sent_prepare macro.

The RBTXN_PREPARE flag (and its corresponding macro) have been renamed
to RBTXN_IS_PREPARE to explicitly indicate the transaction
type. Therefore, this commit also adds the RBTXN_IS_PREPARE flag to
the transaction that is a prepared transaction and has been skipped,
which previously had only the RBTXN_SKIPPED_PREPARE flag.

Reviewed-by: Amit Kapila, Peter Smith
Discussion: https://postgr.es/m/CAA4eK1KgNmBsG%3D155E7QQ6TX9RoWnM4z5Z20SvsbwxSe_QXYsg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/abfb29648f9adcde84656afde1d50bc8b8e9b6e0

Modified Files
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src/backend/replication/logical/proto.c | 2 +-
src/backend/replication/logical/reorderbuffer.c | 50 +++++++++++++++----------
src/backend/replication/logical/snapbuild.c | 2 +-
src/include/replication/reorderbuffer.h | 8 ++--
4 files changed, 38 insertions(+), 24 deletions(-)