require the storage lock in lastTransaction.

This is required (or maybe strongly implied) by the IStorage interface.

Also, rearrange the order of operations in _process_invalidations.

Each of these changes avoid a potential race when a connection starts
a transaction while invalidateTransaction is being called. Basically,
we don't want a connection to get a lastTransaction corresponding to
invalidations it hasn't processed.

This neither fixes nor provokes any test failures. :) The need for
this change was discovered via code inspection. It's hard to see how
to test the race without getting being insanely whitebox.

src/ZEO/ClientStorage.py

@@ -1119,7 +1119,8 @@ class ClientStorage(object):
             self._tpc_cond.release()
 
     def lastTransaction(self):
-        return self._cache.getLastTid()
+        with self._lock:
+            return self._cache.getLastTid()
 
     def tpc_abort(self, txn):
         """Storage API: abort a transaction."""
@@ -1473,11 +1474,12 @@ class ClientStorage(object):
             if oid == self._load_oid:
                 self._load_status = 0
             self._cache.invalidate(oid, tid)
-        self._cache.setLastTid(tid)
 
         if self._db is not None:
             self._db.invalidate(tid, oids)
 
+        self._cache.setLastTid(tid)
+
     # The following are for compatibility with protocol version 2.0.0
 
     def invalidateTrans(self, oids):