Closely track mempool byte total. Add "getmempoolinfo" RPC.

Goal:  Gain live insight into the mempool.  Groundwork for future work
that caps mempool size.

src/txmempool.cpp

@@ -402,6 +402,7 @@ bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry)
         for (unsigned int i = 0; i < tx.vin.size(); i++)
             mapNextTx[tx.vin[i].prevout] = CInPoint(&tx, i);
         nTransactionsUpdated++;
+        totalTxSize += entry.GetTxSize();
     }
     return true;
 }
@@ -426,6 +427,8 @@ void CTxMemPool::remove(const CTransaction &tx, std::list<CTransaction>& removed
             removed.push_front(tx);
             BOOST_FOREACH(const CTxIn& txin, tx.vin)
                 mapNextTx.erase(txin.prevout);
+
+            totalTxSize -= mapTx[hash].GetTxSize();
             mapTx.erase(hash);
             nTransactionsUpdated++;
         }
@@ -477,6 +480,7 @@ void CTxMemPool::clear()
     LOCK(cs);
     mapTx.clear();
     mapNextTx.clear();
+    totalTxSize = 0;
     ++nTransactionsUpdated;
 }
 
@@ -487,9 +491,12 @@ void CTxMemPool::check(CCoinsViewCache *pcoins) const
 
     LogPrint("mempool", "Checking mempool with %u transactions and %u inputs\n", (unsigned int)mapTx.size(), (unsigned int)mapNextTx.size());
 
+    uint64_t checkTotal = 0;
+
     LOCK(cs);
     for (std::map<uint256, CTxMemPoolEntry>::const_iterator it = mapTx.begin(); it != mapTx.end(); it++) {
         unsigned int i = 0;
+        checkTotal += it->second.GetTxSize();
         const CTransaction& tx = it->second.GetTx();
         BOOST_FOREACH(const CTxIn &txin, tx.vin) {
             // Check that every mempool transaction's inputs refer to available coins, or other mempool tx's.
@@ -518,6 +525,8 @@ void CTxMemPool::check(CCoinsViewCache *pcoins) const
         assert(tx.vin.size() > it->second.n);
         assert(it->first == it->second.ptx->vin[it->second.n].prevout);
     }
+
+    assert(totalTxSize == checkTotal);
 }
 
 void CTxMemPool::queryHashes(vector<uint256>& vtxid)