Mythril - Fix bugs related to multi-threading support in db initialization functionality

Mythril is a security analysis tool for Ethereum smart contracts, written in Python.

Recently, multi-threading support was added to the db initialization feature. Users report bugs in the sync process. Your goal is to fix the bugs described in these issue tickets:
https://github.com/ConsenSys/mythril/issues/119
https://github.com/ConsenSys/mythril/issues/110
https://github.com/ConsenSys/mythril/issues/113

Final Submission Guidelines

Submit Git patch against the latest commit in the master branch of Mythril repo (d0a3858400b727bdc5c927487e0dcc69ca1ef14d at the moment of writing), and a brief note on how you have resolved these bugs, and how it should be verified.