Support for Postgres 15

src/decoderbufs.c:503:59: error: no member named 'commit_time' in 'struct ReorderBufferTXN'
  rmsg.commit_time = TIMESTAMPTZ_TO_USEC_SINCE_EPOCH(txn->commit_time);
                                                     ~~~  ^
src/decoderbufs.c:69:47: note: expanded from macro 'TIMESTAMPTZ_TO_USEC_SINCE_EPOCH'
#define TIMESTAMPTZ_TO_USEC_SINCE_EPOCH(t)    t + (POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * USECS_PER_DAY;
                                              ^
src/decoderbufs.c:541:59: error: no member named 'commit_time' in 'struct ReorderBufferTXN'
  rmsg.commit_time = TIMESTAMPTZ_TO_USEC_SINCE_EPOCH(txn->commit_time);
                                                     ~~~  ^
src/decoderbufs.c:69:47: note: expanded from macro 'TIMESTAMPTZ_TO_USEC_SINCE_EPOCH'
#define TIMESTAMPTZ_TO_USEC_SINCE_EPOCH(t)    t + (POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * USECS_PER_DAY;
                                              ^
src/decoderbufs.c:593:59: error: no member named 'commit_time' in 'struct ReorderBufferTXN'
  rmsg.commit_time = TIMESTAMPTZ_TO_USEC_SINCE_EPOCH(txn->commit_time);
                                                     ~~~  ^
src/decoderbufs.c:69:47: note: expanded from macro 'TIMESTAMPTZ_TO_USEC_SINCE_EPOCH'
#define TIMESTAMPTZ_TO_USEC_SINCE_EPOCH(t)    t + (POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * USECS_PER_DAY;
                                              ^
3 errors generated.
make: *** [src/decoderbufs.o] Error 1 

--- a/src/include/replication/reorderbuffer.h
+++ b/src/include/replication/reorderbuffer.h
@@ -297,7 +297,11 @@ typedef struct ReorderBufferTXN
         * Commit or Prepare time, only known when we read the actual commit or
         * prepare record.
         */
-       TimestampTz commit_time;
+       union
+       {
+               TimestampTz commit_time;
+               TimestampTz prepare_time;
+       }                       xact_time;