Retry config: separate max open attempts from backoff factor

[dirkmc]

This PR separates out the maximum number of attempts to open a stream from the exponential backoff factor (the multiplier used when calculating how long to backoff before attempting again).

Also consolidates some retry code used by the networking layers of both retrieval and storage mining into one place.

[codecov-io]

Codecov Report

Merging #467 (f3db88c) into master (aa8533f) will decrease coverage by 0.51%.
The diff coverage is 93.03%.

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
- Coverage   65.58%   65.08%   -0.50%     
==========================================
  Files          46       50       +4     
  Lines        3198     3218      +20     
==========================================
- Hits         2097     2094       -3     
- Misses        875      898      +23     
  Partials      226      226              

Impacted Files	Coverage Δ
shared/retrystream.go	90.91% <90.91%> (ø)
retrievalmarket/network/libp2p_impl.go	85.37% <100.00%> (+4.73%)	⬆️
storagemarket/network/libp2p_impl.go	85.72% <100.00%> (ø)
shared/movekey.go	0.00% <0.00%> (ø)
shared/ready.go	0.00% <0.00%> (ø)
shared/selectors.go	0.00% <0.00%> (ø)

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