SQLCipher database access errors can raise Soledad exceptions. Database access
and multithreading resources are allocated in different places, so we have to
be careful to close all multithreading mechanismis in case of database access
errors. If we don't, zombie threads may haunt the reactor.

This commit adds SQLCipher exception trapping and Soledad exception raising
for database access errors, while properly shutting down multithreading
resources.

From this moment on, we embed a fork of u1db called l2db.

It was breaking E126 and E202 before

Do not initialize the openssl context on each call to decrypt.

I'm not 100% sure of the causal chain, but it seems that the
initialization of the osrandom engine that openssl backend does might be
breaking havoc when sqlcipher is calling rand_bytes concurrently.

further testing is needed to confirm this is the ultimate cause, but in
my tests this change avoids the occurrence of the dreaded Floating Point
Exception in soledad/sqlcipher.

- Resolves: #8180