Jaden/chainstore refactor

[flodesi]

Summary of changes
Changes introduced in this pull request:

• Convert methods in ChainStore that depend only on DB to functions that take DB as a parameter and not self.
• General refactoring to functions that relied on the methods described above.

Closes
#430

[ec2]

Might have to make a bunch of these functions public later. But we can make them public as we need.