Add an option in doc creation APIs to specify a random value for an initial doc revision

Currently the initial revision of a document is deterministic. For instance,
anyone that has created an empty document probably recognizes the revision starting with "1-967a00dff...". In order to account for situations when a document is continually purged and recreated we're going to add randomness to this initial revision by specifying a 0-$rev in the request coordinator. We will then include this in the revision generation but drop the 0-$rev entry from the revision's path.

Thus, the new API will look like this:
acurl -X PUT https://http://adm:pass@127.0.0.1:5984/test-db/newdoc1?rev=0-adfdafa123 -d '{}'
And similarly for _bulk_docs

For a user who wants to create a doc, then purge it, and then re-create, it is recommended to recreate it with another random revision.

It is important to note that the 0-$rev only affects document creation. Once
a document exists, updates to the document will continue to update their hash in the same deterministic fashion. Ie, once a document exists, identical
updates will result in identical revisions.
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The following changes need to be made in the code:
1. API changes to allow to specify random rev in doc PUT requests, _bulk_docs
2. Internals:
2.1 Use a new revision here: https://github.com/apache/couchdb-couch/blob/master/src/couch_db.erl#L886
2.2 Don't include provided 0-$rev entry to the revision's path (find wherever new_revid is called from; could be 2-3 places)
2.3 Reject a 0-$rev during replication