fix: rpc.undo/redo with non-instant confirmations

[iamdefinitelyahuman]

What I did

Handle issues with rpc.undo / redo when using non-instant confirmations with Ganache.

How I did it

Instead of adding the tx's to the undo buffer immediately, they are handled via a thread which waits for the tx to confirm.

How to verify it

Run tests.

[codecov-io]

Codecov Report

Merging #483 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
+ Coverage   88.23%   88.24%   +0.01%     
==========================================
  Files          56       56              
  Lines        6077     6093      +16     
  Branches     1337     1340       +3     
==========================================
+ Hits         5362     5377      +15     
  Misses        494      494              
- Partials      221      222       +1     

Impacted Files	Coverage Δ
brownie/network/account.py	91.81% <100.00%> (+0.11%)	⬆️
brownie/network/rpc.py	88.66% <100.00%> (+0.28%)	⬆️
brownie/network/transaction.py	86.87% <0.00%> (-0.02%)	⬇️

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