Add ability to dump headers or mimetypes for original messages.
Add ability to dump all messages, instead of just hashes listed.
Add new command cmd/headers to import all headers from messages in table
original. Utility can work incrementally or do fully table imports.
Moved 'fetch all original messages' code to db/util.go. Added conditional
version given table name that will only return original messages that do not
have matching hashes in the specified table.
