Generate snapshots as needed. Remove append event subscription

[nhunzaker]

What:

Microcosm History emits an append event that tells its associated Microcosms that it needs to create a snapshot. We can lazily generate snapshots as they are needed, allowing us to get rid of that event.

Why:

I am trying to move internals over to observables, which more accurately describe how Microcosms and Actions work. To do that, I need to eliminate a few events, particularly in History.

Basically I'm trying to get everything to work via a single change event type, which could eventually be replaced with the observable lifecycle:

1. start (setup microcosm hook)
2. next (change microcosm hook)
3. complete (teardown microcosm hook)
4. error (handled via actions)

[codecov-io]

Codecov Report

Merging #438 into master will increase coverage by 7.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #438      +/-   ##
==========================================
+ Coverage    70.2%   77.23%   +7.03%     
==========================================
  Files          44       44              
  Lines        1376     1362      -14     
  Branches      258      251       -7     
==========================================
+ Hits          966     1052      +86     
+ Misses        339      257      -82     
+ Partials       71       53      -18

Impacted Files	Coverage Δ
packages/microcosm/src/history.js	100% <ø> (ø)	:arrow_up:
packages/microcosm/src/microcosm.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/addons/action-button.js	100% <0%> (+100%)	:arrow_up:
packages/microcosm/src/addons/action-queue.js	100% <0%> (+100%)	:arrow_up:
packages/microcosm/src/addons/indexing.js	100% <0%> (+100%)	:arrow_up:

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