upgrade mobile test framework to use CBL blob to replace attachment

With CBL employed blob type as a replacement of the original reserved _attachments keyword, mobile test framework needs to adopt blob in the fundamental doc create utilities and in all test cases. This includes:

• replace doc_body["_attachments"] = data_blah anywhere in the mobile test framework code with set blob
• since the nature of SGW blob support, a blob on CBL will becomes a blob entry + an entry with attachments entry, python code compare_docs need to be rewritten to meet this requirements
• test_delta_sync_replication test case has missing the doc_body reset with the modified value, need to be fixed