### Describe the issue

Restarting bitcoin, mempool starts filling from scratch.

### Can you reliably reproduce the issue?
#### If so, please list the steps to reproduce below:
1. Wait for mempool to reach for example 3000 transactions
2. Restart bitcoin
3. Watch mempool filling slowly from scratch

### Expected behaviour
Mempool would already contain ~3000 TX on startup

### Actual behaviour
It starts at 0 and fills up slowly with a few per second.

### Screenshots.
http://imgur.com/a/or82d

### What version of bitcoin-core are you using?
Windows x64 0.14rc1 binary from https://bitcoin.org/bin/bitcoin-core-0.14.0/test.rc1/


### Any extra information that might be useful in the debugging process.
I may not be aware of how the mempool.dat thing works and there could just be a misunderstanding.

if (pool.exists(hash))
        return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-in-mempool");
    
    std::set<uint256> setConflicts;
    {
    LOCK(pool.cs); 
    BOOST_FOREACH(const CTxIn &txin, tx.vin)
    {
        auto itConflicting = pool.mapNextTx.find(txin.prevout);
        if (itConflicting != pool.mapNextTx.end())
        {
            const CTransaction *ptxConflicting = itConflicting->second;
            if (!setConflicts.count(ptxConflicting->GetHash()))
            {
                
                
                
                
                
                
                
                
                
                
                
                
                bool fReplacementOptOut = true;
                if (fEnableReplacement)
                {
                    BOOST_FOREACH(const CTxIn &_txin, ptxConflicting->vin)