### 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.

{
    for (BlockMap::iterator it = mapBlockIndex.begin(); it != mapBlockIndex.end(); ++it) {
        CBlockIndex* pindex = it->second;
        if (pindex->nFile == fileNumber) {
            pindex->nStatus &= ~BLOCK_HAVE_DATA;
            pindex->nStatus &= ~BLOCK_HAVE_UNDO;
            pindex->nFile = 0;
            pindex->nDataPos = 0;
            pindex->nUndoPos = 0;
            setDirtyBlockIndex.insert(pindex);
            
            
            
            
            std::pair<std::multimap<CBlockIndex*, CBlockIndex*>::iterator, std::multimap<CBlockIndex*, CBlockIndex*>::iterator> range = mapBlocksUnlinked.equal_range(pindex->pprev);
            while (range.first != range.second) {
                std::multimap<CBlockIndex *, CBlockIndex *>::iterator _it = range.first;
                range.first++;
                if (_it->second == pindex) {
                    mapBlocksUnlinked.erase(_it);
                }
            }
        }
    }
    vinfoBlockFile[fileNumber].SetNull();
    setDirtyFileInfo.insert(fileNumber);
}
void UnlinkPrunedFiles(std::set<int>& setFilesToPrune)