Implement tx confirmation for channelOpen

[andrevmatos]

Part of #613
Todo in a later PR, is to extend this method to all on-chain transactions.
First time the on-chain transaction is seen, it gets dispatched with payload.confirmed property present but set as undefined, meaning pending. Reducer will persist/serialize it to state.pendingTx (thanks to #759).

Then, on each block, the possibly confirmed actions (when more than config.confirmationBlocks have passed since first/pending action) gets fetched with getTransactionReceipt, and if the transaction is still present on-chain, it gets re-emitted with confirmed=true, which also removes it from the pendintTx state.
If it isn't present, on each block until 2 * confirmationBlocks, check again. If it still isn't there/confirmed after this timeout, consider the transaction as having been removed by a chain reorg, and re-emit it, now with confirmed=false, which also removes it from pendingTx and rejects any pending promise.
.
The confirmable actions always being emitted twice (first as pending, later as confirmed or removed) enables each part of the codebase to chose which actions/events to react to, be it first time seeing a tx event, or upon confirmation: e.g. the transfer epics can react early to channelClose and avoid accepting new transfers or mediating (in the future), while accepting deposits as increasing the channel's capacity is done only after it has been confirmed.

The confirmationEpic reacting on state ensures this algorithm is safe upon shutdown and later client restart, even with to-be-confirmed actions being able to be picked up again after much time (as long as receipt can be fetched by eth client, which may not be the case without full or even archiving node with enormous block ranges, but we'd consider this as an unexpected edge case).

[codecov]

Codecov Report

Merging #1009 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
- Coverage   96.04%   96.00%   -0.04%     
==========================================
  Files          95       95              
  Lines        3588     3633      +45     
  Branches      759      775      +16     
==========================================
+ Hits         3446     3488      +42     
- Misses         85       88       +3     
  Partials       57       57              

Flag	Coverage Δ
#dapp	91.46% <ø> (ø)	⬆️
#sdk	97.86% <95.31%> (-0.09%)	⬇️

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[kelsos]

looks good, thank you 👍

[nephix]

Tests must have been pain. Good job 👍