Functional SoledadBackedAccount and LeapMailboxes

The imap service is launched from the tac file,
and still needs some information to be provided in separate
config files that stub much of the initialization parameters.

working fetch and store methods.
tested with offlineimap and thunderbird.
several mailboxes might be broken.
cleanup and complete docs
use the same soledad instance

for incoming and mailbox
Merge remote-tracking branch 'kali/feature/imap-service2' into develop

Conflicts:
	pkg/requirements.pip
	src/leap/mail/imap/tests/test_imap.py
provide a initialization entrypoint for client use
Merge remote-tracking branch 'kali/bug/imap-service-use-defer-to-thread' into develop
Adapt smtp relay to latest soledad and keymanager.
Add .gitignore file.
Adapt IMAP to latest Soledad api.

* Also fix some tests that where not up-to-date with code.
Fix setuptools' test suite spec.
Add dependency for leap.keymanager.
Merge remote-tracking branch 'drebs/feature/3037-separate-keymanager-into-its-own-package' into develop
Fix keymanager requirement to version 0.2.0.
Merge remote-tracking branch 'drebs/bug/3264-fix-keymanager-version' into develop
Add client certificate authentication
Merge remote-tracking branch 'chiiph/feature/cert_auth' into develop
use the right import path
refactor recurring fetch
Merge remote-tracking branch 'kali/feature/refactor_imap_fetch' into develop
catch exception if tried to stop not running loop