Enable faster channel opening & deposit by parallelizing them and their confirmations

[andrevmatos]

Currently, channel open & deposit can take a very long time, since they're called separatelly from userspace and needs to be processed serially, as deposit can only be done after channel state is in place.
But the information needed for deposit is available while opening the channel (when the respective txs are sent and mined), and therefore, we can parallelize these operations (and specially their confirmation) to minimize the time needed to open a channel.
So, this PR:

• makes channelOpen.request accept an optional deposit param
• if requesting a deposit with said param, call token.approve in parallel with openChannel
• wait for both to be mined in parallel
• iff channel open was successful, wait for the channelId and right away call setTotalDeposit with it
• emits respective channelDeposit.failure if it doesn't succeed
• public openChannel method accepts deposit, waits for the respective events, and notify users through an onChange callback
• adapt dApp to use above param and callback instead of slow separate open + deposit calls

With this, open + deposit can complete and be confirmed as soon as N + 1 blocks, while the previous schema required 2 * (N + 1) blocks, where N is the number of confirmation blocks.

[codecov]

Codecov Report

Merging #1128 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
- Coverage   95.06%   95.01%   -0.05%     
==========================================
  Files         111      111              
  Lines        3972     4013      +41     
  Branches      807      827      +20     
==========================================
+ Hits         3776     3813      +37     
- Misses        154      157       +3     
- Partials       42       43       +1     

Flag	Coverage Δ
#dapp	88.03% <50.00%> (-0.12%)	⬇️
#sdk	97.80% <95.23%> (-0.08%)	⬇️

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[nephix]

Looks good thank you