Implement cache layer for file storage

This was mentioned in #234.
Disk operations are not so great on Windows/Android systems.
So we can add a disk cache from our side to collect write operations and flush them once buffer if filled up or periodically.

This one is implemented in libtorrent. Some interesting points there: https://github.com/arvidn/libtorrent/blob/master/src/disk_io_thread.cpp#L874 and https://github.com/arvidn/libtorrent/blob/master/src/disk_io_thread.cpp#L1679 .
Not sure we need that level of complexity.
I was looking at golang's bufio implementation (https://golang.org/src/bufio/bufio.go?s=14456:14486#L599) which is pretty simple. We just miss offset writes, and, probably, need to collect multiple consequent chunks into one bigger write operation.

Probably have to manage flushing if need to verify the piece or something.