Bug report and fix about building historic snapshot

Hello, I find a bug in building historic snapshot and the steps to reproduce are as follows:

Prepare:

(pub)create table t1 (id int primary key);

(pub)insert into t1 values (1);

(pub)create publication pub for table t1;

(sub)create table t1 (id int primary key);

Reproduce:

(pub)begin; insert into t1 values (2); (txn1 in session1)

(sub)create subscription sub connection 'hostaddr=127.0.0.1 port=5432 user=xxx dbname=postgres' publication pub; (pub will switch to BUILDING_SNAPSHOT state soon)

(pub)begin; insert into t1 values (3); (txn2 in session2)

(pub)create table t2 (id int primary key); (session3)

(pub)commit; (commit txn1, and pub will switch to FULL_SNAPSHOT state soon)

(pub)begin; insert into t2 values (1); (txn3 in session3)

(pub)commit; (commit txn2, and pub will switch to CONSISTENT state soon)

(pub)commit; (commit txn3, and replay txn3 will failed because its snapshot cannot see t2)

Reasons:
We currently don't track the transaction that begin after BUILDING_SNAPSHOT
and commit before FULL_SNAPSHOT when building historic snapshot in logical
decoding. This can cause a transaction which begin after FULL_SNAPSHOT to take
an incorrect historic snapshot because transactions committed in BUILDING_SNAPSHOT
state will not be processed by SnapBuildCommitTxn().

To fix it, we can track the transaction that begin after BUILDING_SNAPSHOT and
commit before FULL_SNAPSHOT forcely by using SnapBuildCommitTxn().

--
Regards,
ChangAo Chen