Use cookie authentication during replication

    In `couch_replicator_httpc:setup/1`, if the URL contains basic auth
    credentials, the replicator now removes those credentials from the URL
    and inserts them into a new ets table, whose identifier is stored in the
    returned `httpdb` record in a new field named `credentials`.
    
    Then, if during replication there's a HTTP status code 401, the
    replicator does a POST-request to the remote DB's `/_session` endpoint,
    in which the request body contains the user name and password from the
    basic auth credentials stored in the ets table.  If that request is
    successful, the session cookie is inserted into the ets table together
    with the current timestamp returned by `os:timestamp/0`.  From that
    point on, `couch_replicator_httpc:send_req/3` always retrieves the
    cookie from the ets table and sets it as value in the "Cookie" request
    header.  When a new session cookie is set, the cookie value, together
    with a new timestamp, will be updated in the ets table accordingly.
    Now, after having started a new session, the request that generated the
    response status code 401 is repeated.
    
    In order to prevent the replicator from wrongly starting a new session
    after getting a 401 status code and then repeating the request again
    (and thereby causing an infinite loop), it is checked whether the
    difference between the current timestamp returned by `os:timestamp/0`
    and the timestamp stored together with the session cookie is greater
    than one minute.  A new session is started and the request is repeated
    only if the difference is greater than one minute.  Otherwise it is
    proceeded as ever, i.e., the response with the 401 status code is
    returned.
There's another consideration that I forgot about, until Robert Kowalski
brought it back to my awareness in a conversation we had yesterday.

Using an ETS table for keeping credentials has the important benefit of
avoiding that those will appear in stack traces.  Although one could use
the `sensitive` process flag to achieve the same, we probably don't want
to do that, as it entails disadvantages with regard to tracing and
debugging the respective process (see
http://www.erlang.org/doc/man/erlang.html#process_flag-2).