[feat] use one doc per remote replica in sync log

Before this change, we used a complicated update handler for storing the sync
state on the couchdb backend. That update handler was implemented as an
attempt to make couchdb take care of some validation for the update of the
sync log during the sync exchange, mainly to allow concurrent received
documents insertion during a sync.

Right now we rely on the remote sending one document at a time and do not
support concurrent insertions in the remote database backed by couch. Because
of that, the code removed by this commit was unneeded. And more: it was a
bottleneck of the sync process because we were writing to an unique file and
using unnecessary couch design docs processing for that. So this commit both
simplifies the storage of remote sync and removes a bottleneck of the sync
process.

Conflicts:
	common/src/leap/soledad/common/couch.py
	common/src/leap/soledad/common/tests/test_couch.py

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