remove taskthread dependency
use max cpu_count workers on pool
reorganize pragmas, stub SQLCipherOptions object
move public api to its own file
Refactor soledad api to use async db

* add examples and benchmarks
* remove autocommit mode, allow wal disabling
* lock initialization
* make api use async calls
Separate soledad interfaces

* Separate local storage, syncers and shared_db
* Comment out unused need_sync method
* Use twisted LoopingCall
* Create a threadpool for syncs
* Return deferred from sync method
* Do not pass crypto to SQLCipherDatabase
* Pass replica_uid to u1db_syncer
* Rename / reorganize some initialization methods
Adapt sqlcipher tests to new api.
working sync-threadpool

* Completed mapping of async dbpool
* Fixed shared db initialization.

Stuff To Be Fixed yet:

[ ] All inserts have to be done from the sync threadpool
[ ] Correctly handle the multiprocessing pool, and the callback
    execution.
[ ] Register a shutdown call for closing the sync sqlcipher accessor