Panic when putting document with duplicate non-imported doc ID

When attempting to add a document through sync gateway when a non-imported document with a matching ID already exists in Couchbase Server a panic occurs. Sync Gateway recovers from this panic and continues to operate, however, this should be fixed and instead return a document conflict error.

Reproduction steps:

1. Start an SG instance with import disabled and connect to CB Server.
2. Create a document in Couchbase Server, eg. ID 'test'.
3. Attempt to PUT a document via Sync Gateway with ID 'test'.
4. Sync Gateway will fail to respond to the PUT request and will panic.