Import U1DB code into Soledad repository and remove dependency of U1DB package
There are some reasons why Soledad should not depend on u1db package:
- u1db is not maintained.
- soledad use of u1db has diverged from original u1db intent. For example, crypto while syncing adds aditional processing needs.
- development will be easier if we refactor to remove some intermediate classes and simplify inheritance.
- development will be easier if we don't need to maintain strict dependency on u1db and are free to modify all the code as fit.
- soledad server needs a better implementation that will make use of real parallelism.
(from redmine: created on 2016-06-21, closed on 2016-07-26)