If there's no limit to the number of concurrent blob writes in the
server, the maximum limit of open files will eventually be reached, and
the processing of requests will start crashing. This commit adds
a semaphore to limit the number of concurrent writes in the server.

Related: #8973

An errback was missing in the PUT renderer method of the incoming API.
Because of that, requests to that endpoint were not being correctly
finished in case of errors when writing blobs. That was causing delivery
requests to hang until timeout.

Closes: #8977

As kali pointed out, one can disable blobs after enabling it, which
would cause data loss as blobs documents would become unreacheable. This
commit adds a warning and refuses to start the server.

-- Resolves: #8866

Introduction of local services authentication added a configuration file
containing the auth tokens for each service. There were different names
for that file, and this commit standardizes all of them to the same
value: /etc/soledad/services.tokens

Soledad Server was previously using something in /srv to store blobs in
the server side. Debian/lintian doesn't like that at all, so we are
changing to /var/lib/soledad/blobs.

Closes: #8948