Previously we assign it as max_prepared_xacts. It is used to initialize some
2pc related shared memory. For example the array shmCommittedGxactArray is
created with this length and that array is used to collect not-yet "forgotten"
distributed transactions during master/standby recovery, but the array length
might be problematic since:

1. If master max_prepared_xacts is equal to segment max_prepared_xacts as
usual.  It is possible some distributed transactions use just partial gang so
the total distributed transactions might be larger (and even much larger) than
max_prepared_xacts. The document says max_prepared_xacts should be greater than
max_connections but there is no code to enforce that.

2. Also it is possible that master max_prepared_xacts might be different than
segment max_prepared_xacts (although the document does not suggest it there is
no code to enforce that),

To fix that we use MaxBackends for the gxact number on master. We may just use
guc max_connections (MaxBackends includes number for autovacuum workers and bg
workers additionally besides guc max_connections), but I'm conservatively using
MaxBackends,  since this issue is annoying - standby can not recover due to the
FATAL message as below even after postgres reboot unless we temporarily
increase the guc max_prepared_transactions value.

2020-07-17 16:48:19.178667
CST,,,p33652,th1972721600,,,,0,,,seg-1,,,,,"FATAL","XX000","the limit of 3
distributed transactions has been reached","It should not happen. Temporarily
increase max_connections (need postmaster reboot) on the postgres (master or
standby) to work around this issue and then report a bug",,,,"xlog redo at
0/C339BA0 for Transaction/DISTRIBUTED_COMMIT: distributed commit 2020-07-17
16:48:19.101832+08 gid = 1594975696-0000000009, gxid =
9",,0,,"cdbdtxrecovery.c",571,"Stack trace:

1    0xb3a30f postgres errstart (elog.c:558)
2    0xc3da4d postgres redoDistributedCommitRecord (cdbdtxrecovery.c:565)
3    0x564227 postgres <symbol not found> (xact.c:6942)
4    0x564671 postgres xact_redo (xact.c:7080)
5    0x56fee5 postgres StartupXLOG (xlog.c:7207)
Reviewed-by: Nxiong-gang <gxiong@pivotal.io>