To prevent distributed deadlock, in Greenplum DB an exclusive table lock is
held for UPDATE and DELETE commands, so concurrent updates the same table are
actually disabled.

We add a backend process to do global deadlock detect so that we do not lock
the whole table while doing UPDATE/DELETE and this will help improve the
concurrency of Greenplum DB.

The core idea of the algorithm is to divide lock into two types:

- Persistent: the lock can only be released after the transaction is over(abort/commit)
- Otherwise cases

This PR’s implementation adds a persistent flag in the LOCK, and the set rule is:

- Xid lock is always persistent
- Tuple lock is never persistent
- Relation is persistent if it has been closed with NoLock parameter, otherwise
  it is not persistent Other types of locks are not persistent

More details please refer the code and README.

There are several known issues to pay attention to:

- This PR’s implementation only cares about the locks can be shown
  in the view pg_locks.
- This PR’s implementation does not support AO table. We keep upgrading
  the locks for AO table.
- This PR’s implementation does not take networking wait into account.
  Thus we cannot detect the deadlock of GitHub issue #2837.
- SELECT FOR UPDATE still lock the whole table.
Co-authored-by: NZhenghua Lyu <zlv@pivotal.io>
Co-authored-by: NNing Yu <nyu@pivotal.io>