Fix default CSP setting for Ace Editor

Like @sebastianrothbucher noticed in apache/couchdb-fauxton#5
the Ace editor needs base64 image data as image source for their
icons
Expose and add helper for tests

In order to make them reusable in the chhtpd integration-tests.
Additionally adding a post and delete helper.

COUCHDB-2462
Move db:open_db_file into db_updater
Create db_updater:delete_db_file
Don't try to remove '.compact' file twice on db deletion
Introduce private db_updater:delete_compaction_files to remove duplication
Add test case for doc_from_multi_part_stream
Move doc_from_multi_part_stream into httpd_multipart
Extract parse_multipart_stream from doc_from_multi_part_stream
Move simplified version of doc_from_multi_part_stream back to couch_doc
Add test case for doc_to_multi_part_stream
Extract encode_multipart_stream function out of doc_to_multi_part_stream
Move fetching of atts from atts_to_mp into doc_to_multipart_stream
Do not calculate AttFun for every att since we use same function for all atts in the doc
Remove unnecessary branching
Move encode_multipart_stream from couch_doc into couch_httpd_multipart
Rename parse_multipart_stream into decode_multipart_stream for consistency with encode_multipart_stream
Rename abort_multi_part_stream into abort_multipart_stream for consistency
Extract decode_attributes frunction from doc_to_multi_part_stream
Add doc:len_doc_to_multi_part_stream test