### 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.

{
    {
        LOCK(cs_main);
        for (const CBlockHeader& header : headers) {
            CBlockIndex *pindex = NULL; 
            if (!AcceptBlockHeader(header, state, chainparams, &pindex)) {
                return false;
            }
            if (ppindex) {
                *ppindex = pindex;
            }
        }
    }
    NotifyHeaderTip();
    return true;
}
static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock)
{
    const CBlock& block = *pblock;
    if (fNewBlock) *fNewBlock = false;
    AssertLockHeld(cs_main);
    CBlockIndex *pindexDummy = NULL;
    CBlockIndex *&pindex = ppindex ? *ppindex : pindexDummy;