### 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 (pos.IsNull())
        return NULL;
    boost::filesystem::path path = GetBlockPosFilename(pos, prefix);
    boost::filesystem::create_directories(path.parent_path());
    FILE* file = fopen(path.string().c_str(), "rb+");
    if (!file && !fReadOnly)
        file = fopen(path.string().c_str(), "wb+");
    if (!file) {
        LogPrintf("Unable to open file %s\n", path.string());
        return NULL;
    }
    if (pos.nPos) {
        if (fseek(file, pos.nPos, SEEK_SET)) {
            LogPrintf("Unable to seek to position %u of %s\n", pos.nPos, path.string());
            fclose(file);
            return NULL;
        }
    }
    return file;
}
FILE* OpenBlockFile(const CDiskBlockPos &pos, bool fReadOnly) {
    return OpenDiskFile(pos, "blk", fReadOnly);
}
FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly) {
    return OpenDiskFile(pos, "rev", fReadOnly);
}
boost::filesystem::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix)