### 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 (int fileNumber = 0; fileNumber < nLastBlockFile; fileNumber++) {
            nBytesToPrune = vinfoBlockFile[fileNumber].nSize + vinfoBlockFile[fileNumber].nUndoSize;
            if (vinfoBlockFile[fileNumber].nSize == 0)
                continue;
            if (nCurrentUsage + nBuffer < nPruneTarget)  
                break;
            
            if (vinfoBlockFile[fileNumber].nHeightLast > nLastBlockWeCanPrune)
                continue;
            PruneOneBlockFile(fileNumber);
            
            setFilesToPrune.insert(fileNumber);
            nCurrentUsage -= nBytesToPrune;
            count++;
        }
    }
    LogPrint("prune", "Prune: target=%dMiB actual=%dMiB diff=%dMiB max_prune_height=%d removed %d blk/rev pairs\n",
           nPruneTarget/1024/1024, nCurrentUsage/1024/1024,
           ((int64_t)nPruneTarget - (int64_t)nCurrentUsage)/1024/1024,
           nLastBlockWeCanPrune, count);
}
bool CheckDiskSpace(uint64_t nAdditionalBytes)
{
    uint64_t nFreeBytesAvailable = boost::filesystem::space(GetDataDir()).available;
    
    if (nFreeBytesAvailable < nMinDiskSpace + nAdditionalBytes)
        return AbortNode("Disk space is low!", _("Error: Disk space is low!"));
    return true;
}
FILE* OpenDiskFile(const CDiskBlockPos &pos, const char *prefix, bool fReadOnly)