`MyProc->inCommit` is to protect checkpoint running during inCommit
transactions.

However, `MyProc->lxid` has to be valid because `GetVirtualXIDsDelayingChkpt()`
and `HaveVirtualXIDsDelayingChkpt()` require `VirtualTransactionIdIsValid()` in
addition to `inCommit` to block the checkpoint process.

In this fix, we defer clearing `inCommit` and `lxid` to `CommitTransaction()`.