TitanDBImpl::Open only open base DB once (#114)

Summary:
Refactor `TitanDBImpl::Open` to open base DB only once. Previously we open the DB twice - the first time only to obtain `cf_id` for each CF, and then initialize internals before open base DB for real. The patch change to open base DB only once, initialize internals after that. Before `TitanDBImpl` is initialized, `BaseDbListener` will be no-op and `TableBuilder` will not output blob files.

Test Plan:
Added new tests