Fix baseapp ante handler simulation mode bug

[jaekwon]

Before, app.initializeContext was ignoring the ctx's MultiStore and setting a new one from the app, which was making things pretty confusing.

Also, before, context.multiStore() used to be unexposed, but there's no real need to because MultiStore doesn't expose the keys. So this is now exposed to make the PR here possible and simplify the baseapp logic a bit.

If context.MultiStore() were the CommitMultiStore, there is an least-authority security issue, because the CommitMultiStore exposes more than is safe for ante-handlers and handlers to work with, but with the current BaseApp implementation, this is not an issue as the context.MultiStore() passed in to ante-handler, handler, beginBlocker, and endBlocker are all CacheMultiStores.

Related issue: #2867
Related issue: #2824

[sunnya97]

@jaekwon Why do we want it to use the checkTx context. It should be querying the latest committed block, not the local checkTx state.

[cwgoes]

Another cool thing would be to simulate transactions and then run queriers against the simulated state to see what changes occurred.

[codecov]

Codecov Report

Merging #2847 into develop will decrease coverage by 0.08%.
The diff coverage is 89.47%.

@@             Coverage Diff             @@
##           develop    #2847      +/-   ##
===========================================
- Coverage    57.04%   56.96%   -0.09%     
===========================================
  Files          120      131      +11     
  Lines         8277     8582     +305     
===========================================
+ Hits          4722     4889     +167     
- Misses        3240     3363     +123     
- Partials       315      330      +15

[jaekwon]

Merging for 9002.