Accessing attachments created in 2.x from a 1.x client

[djpongh]

Originally: https://github.com/couchbase/couchbase-lite-android/issues/1415

Need a test to verify:
As a user I would like to attachments created via a 2.0 client to be replicated to any 1.x clients connected to the cluster.

Need to verify that 2.0 apps that need backward compatibility can create blobs in an “_attachments” property so that they are recognized by older versions.

[raghusarangapani]

Assigning to @sridevi-15 since she is already working on adding replication tests.