Improve basic auth credentials handling in replicator

[nickva]

Previously, there were two ways to pass in basic auth credentials for
endpoints -- using URL's userinfo part and encoding the them in an
"Authorization": "basic ..." header. Neither one is ideal for these reasons:

• Passwords in userinfo doesn't allow using ":", "@" and other characters.
  However, even after switching to always unquoting them like we did recently
  [1], would break authentication for usernames or passwords previously
  containing "+" or "%HH" patterns, as "+" might now be decoded to a " ".

• Base64 encoded headers need an extra step to encode them. Also, quite often
  these encoded headers are confused as being "encrypted" and shared in a
  clear channel.

To improve this, revert the recent commit to unquote URL userinfo parts to
restore backwards compatibility, and introduce a way to pass in basic auth
credentials in the "auth" object. The "auth" object was already added a while
back to allow authentication plugins to store their credentials in it. The
format is:

   "source": {
       "url": "https://host/db",
       "auth": {
           "basic": {
               "username":"myuser",
               "password":"mypassword"
           }
       }
   }

{"auth" : "basic" : {...}} object is checked first, and if credentials are
provided, they will be used. If they are not then userinfo and basic auth
header will be parsed.

Internally, there was a good amount duplication related to parsing credentials
from userinfo and headers in replication ID generation logic and in the auth
session plugin. As a cleanup, consolidate that logic in the
couch_replicator_utils module.

[1] f672b91

The original idea for this scheme belongs to @jaydoane

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation (will create a docs pr after the review)

[jaydoane]

This looks great to me! Only found one minor formatting issue. Tests pass:

$ make eunit apps=couch_replicator
...
=======================================================
  All 301 tests passed.