blockstore: browser fallback to indexdb via bdb

[pinheadmz]

This PR adds an index-browser.js file to blockstore/ that creates LevelBlockStore for chainDB, which is browser compatible.

[pinheadmz]

This is currently working fine in my browser-app full node:
https://github.com/pinheadmz/mobilebcoin

[codecov-io]

Codecov Report

Merging #757 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #757      +/-   ##
==========================================
- Coverage   56.64%   56.61%   -0.03%     
==========================================
  Files         112      112              
  Lines       28140    28140              
  Branches     4800     4800              
==========================================
- Hits        15939    15931       -8     
- Misses      12201    12209       +8

Impacted Files	Coverage Δ
lib/mempool/fees.js	59.42% <0%> (-1.91%)	⬇️

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[braydonf]

Another alternative is for the browser to use SPV, as it doesn't require a blockstore.

[braydonf]

There are number of reasons why a full or pruned node (that requires a blockstore) in a browser (and without a fs) isn't practical:

• It's going to be very slow, as far I know this hasn't been tested on mainnet.
• Persistence issues and limits on storage.
• Various security concerns, browser extensions, private key phishing and etc.
• Can't connect to the network directly.

In the other cases, such as mobile applications with WebView, there will likely be filesystem support, and is for the most part without the above issues.

[tynes]

I don't think that we should deprecate browser support, especially because there is an easy fix to make it work in the browser again