Move call to GetTopTransactionId() earlier in LockAcquire(),

Move call to GetTopTransactionId() earlier in LockAcquire(),
removing an infrequently occurring race condition in Hot Standby.
An xid must be assigned before a lock appears in shared memory,
rather than immediately after, else GetRunningTransactionLocks()
may see InvalidTransactionId, causing assertion failures during
lock processing on standby.

Bug report and diagnosis by Fujii Masao, fix by me.