Txdb tests and updates

[nodech]

This PR attempts to cover balance tracking with tests and fix some edge cases.
This is based on the initial idea of #780 and tests from it.

But expands scope to cover all balances instead of lock/unlocked.
To do full review on the txdb balance states and cover all missing cases, I have created psuedo code like document that includes all the cases this PR covers and future updates that may happen to the txdb methods. All cases that are covered in this PR are marked as checkboxes in the document: https://gist.github.com/nodech/c943e7c50b300b0505cc9dfcae243d48

Suggestions to where to move this document or if it's useful to other developers are welcome here or on the document.

Next tasks (Not part of the PR), maybe create issues:

• Add 'missed credit' events

[coveralls]

coverage: 68.686% (+0.03%) from 68.658% when pulling 2d5c25a on nodech:txdb-updates into 5c287c9 on handshake-org:master.

[nodech]

Updated the document with the stuff that has been verified/implemented: https://gist.github.com/nodech/c943e7c50b300b0505cc9dfcae243d48

[nodech]

Rebased on master to fix bdb segfaults.

[rithvikvibhu]

Tested this branch with a few live wallets of a couple of hundred txs and coins - balance remained same.

Went through the changes (and understand most of it I think/hope), nice work!

P.S. Every attempt at reading test/wallet-balance-test.js, I get lost in some place or another 😄 must've been a pain to write and test

[nodech]

P.S. Every attempt at reading test/wallet-balance-test.js, I get lost in some place or another 😄 must've been a pain to write and test.

Yeah, I get lost when I get back to it for couple minutes as well. But otherwise it would have been huge file. Maybe I will try to simplify at some point. But for now it is what it is.