fix: DDL locking session hanging (#90)

[imranzaheer612]

Unlock the relation if locked by toplevel transaction and a queued DDL command waiting to be replayed.

Issue #90,

As described in the issue if we try to insert in the same table which is going to be drooped by the Node 1. The insert command will apply a lock on the table, as the result when Node 2 will try to replay (dropping the same rel) it will not and will go under a race condition.

I tried unlocking the relation and then locking it again after the DDL is replayed. It worked. But I am not sure weather unlocking a relation which was locked by the toptransaction is safe or not. So let me know if the approach looks OK to you. Thanks

Present behavior.

Node 1:

app=# begin;
BEGIN
app=*# drop table foo;
DROP TABLE

Node 2:

app=# insert into foo values (1);

Hanging.

Node 1:

app=*# commit;
COMMIT

Node 2

app=# insert into foo values (1);
ERROR:  relation 34120 deleted while still in use