Also reduced hash function to 256 bits because our target security level
is now 128 bits.

https://sourceforge.net/p/briar/bugs/49/
Fixed by ensuring that the generated contact and group intents are uniquely identified

bug#49 Fixed by ensuring that intents for different contacts/groups can be distinguished from each other when resolving PendingIntent refs

1. The things we're really trying to protect - contact identities,
message contents, etc - can't be erased from memory because they're
encapsulated inside objects we don't control.

2. Long-term secrets can't be protected by erasing them from memory
because they're stored in the database and the database key has to be
held in memory whenever the app's running.

3. If the runtime uses a compacting garbage collector then we have no
way to ensure an object is erased from memory.

4. Trying to erase secrets from memory makes the code more complex.

Conclusion: Let's not try to protect secrets from an attacker who can
read arbitrary memory locations.

- replaced use of Object instance mutex with a private final Lock object
- replaced Object signaling with specific condition signalling