support parallel for read operations

[siddontang]

@ngaut @c4pt0r

Now all operations including read is serialization, this does not use the advantage of rocksdb.

If the data is expired in read, we will not delete it immediately, but put it in the expired list then delete it in another thread. So that we cau use read mutex for read and write mutex for write.

[ngaut]

Let's make it works well first. Then make it faster.

[siddontang]

Yes, I know, this will be a later optimization. :-)