Major: move to a 2^128 file id space.

The long random id used to identify each file is replaced by an instance of
FileId. Currently, a FileId consists in two longs, which increases the state
space to 2^128. This should be more than enough
(see https://en.wikipedia.org/wiki/Birthday_attack), but this is now modifiable
if needed as the FileId is mostly opaque.
Merge branch 'master' into longer-file-id

Integrate modified comment by Philipp but keep the FileId.
Conflicts:
	src/org/syncany/database/PartialFileHistory.java
Merge branch 'master' into longer-file-id
Move FileId to a byte based storage.

This is a compromise between the full ByteArray class and the special
two long representation.
Merge branch 'longer-file-id' of https://github.com/fabrice-rossi/syncany