Merge pull request #85 from vikramrajkumar/btsx

Merge upstream
When flushing the cached_level_map to the underlying level_map, write data in an leveldb::WriteBatch (hoping for improved performance)
Use const references rather than making copies when flushing
cached_level_map
Fix Mac build
Add more debug logging of pending transactions to help debug why transactions are getting 'stuck'
Added compute_item_hashes utility
Add missing file (oops)
Fix clang build
Prepare for upcoming downstream prefix rename
Update submodules
Merge branch 'develop' into btsx

Conflicts:
	programs/web_wallet
Update web_wallet submodule
Update checkpoint