Add missing lock for mempool.cs.
Add missing lock for mempool.cs.

When CTxMemPool::addUnchecked requires holding mempool.cs.  This was
violated in the unit test, fix it.

pataquets/namecoin-core,namecoin/namecoin-core,namecoin/namecore,namecoin/namecore,domob1812/namecore,namecoin/namecore,domob1812/namecore,namecoin/namecoin-core,namecoin/namecore,pataquets/namecoin-core,pataquets/namecoin-core,namecoin/namecoin-core,namecoin/namecore,namecoin/namecore,domob1812/namecore,pataquets/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,domob1812/namecore,namecoin/namecoin-core,domob1812/namecore,domob1812/namecore,namecoin/namecoin-core,namecoin/namecoin-core