Modernize explorer backend

[LeeSmet]

The current explorer module, which is the backend for explorer pages, is lacking some rather important features, and has some design flaws. It should be adapted to include the following:

• pagination (currently some queries on tfchain return excess of 50MB of data)
• websockets, for real time server -> client updates
• A properly abstracted database layer. Right now database code is interleaved with the regular code. This causes problems, as it is now really hard to switch out DB's (should this be desired). It also prevents us form (manually) keeping certain buckets in memory, causing significant read amplification (no longer required because of plans in R&D merge of explorer/consensus modules #605)

Given all of the above, it might also be worth to create a new explorer module next to the existing one, and replace the old explorer in its entirety once this is done. (more or less the plan as described in #605)

Interesting to investigate: https://github.com/asdine/storm
Also interesting: https://github.com/cayleygraph/cayley

[DylanVerstraete]

Related issue threefoldtecharchive/rivine-chain-explorer#8

[GlenDC]

Relates to #605, as work to be done after.