Fix crashes on multi thread calls to read/write/destroy

[pilgr]

Multi thread access to assertInit was causing race conditions on creating Paper dir which caused exception like RuntimeException("Couldn't create Paper dir:..). That was easy fixed with adding synchronized to the function.

The other problem raised after that: what happened if destroy is called while read/write still in progress? That caused many issues on reading/writing to file. Since destroy makes changes for all keys, and read/write synchronized by particular key, I've introduced a new global Semaphore to lock all per-key operations while global operation is in progress. Also global operations (like destroy and getAllKeys) won't be started before currently running per-key operations (read, write etc) are completed.

Fix #66
Fix #108
Fix #114
Fix #159
Fix #160

[wassil]

LGTM, thank you for doing this!