### 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.

{
    int64_t start = GetTimeMicros();
    std::map<uint256, CAmount> mapDeltas;
    std::vector<TxMempoolInfo> vinfo;
    {
        LOCK(mempool.cs);
        for (const auto &i : mempool.mapDeltas) {
            mapDeltas[i.first] = i.second.second;
        }
        vinfo = mempool.infoAll();
    }
    int64_t mid = GetTimeMicros();
    try {
        FILE* filestr = fopen((GetDataDir() / "mempool.dat.new").string().c_str(), "w");
        if (!filestr) {
            return;
        }
        CAutoFile file(filestr, SER_DISK, CLIENT_VERSION);
        uint64_t version = MEMPOOL_DUMP_VERSION;
        file << version;