Optional In-memory storage of all the data in current version

[kushti]

Make a flag in database constructor to store all the data (current version only) in memory (guarantee is not needed, so if it is impossible to store the whole dataset in memory, dump it on disk anyway).

[jankotek]

Journal (last N updates) should be cached onheap. That also speed up Sharding Task, since it does not have to merge Journal

[jankotek]

Memory mapped files in 0.5 should help it.