gs1.1: use string peer id consistently

[wemeetagain]

Replace many uses of PeerStreams with string peer id as the primary/sole peer identifier

• internal maps, use string peer id where possible
• internal methods, use string peer id where possible
• only use peerStreams when the peerStreams.protocol or peerStreams.write is needed.

In all cases, the variable name id is used to refer to a string peer id.
In a separate PR, we might consider renaming this variable to p to more closely conform to the go-libp2p-pubsub implementation.

[vasco-santos]

LGTM!
This is definitely a good call 👍

[codecov-commenter]

Codecov Report

Merging #111 into gsv1.1 will increase coverage by 0.54%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           gsv1.1     #111      +/-   ##
==========================================
+ Coverage   75.38%   75.93%   +0.54%     
==========================================
  Files          22       22              
  Lines        1361     1342      -19     
==========================================
- Hits         1026     1019       -7     
+ Misses        335      323      -12     

Impacted Files	Coverage Δ
src/pubsub.js	82.26% <0.00%> (-0.83%)	⬇️
src/index.js	68.26% <0.00%> (+0.81%)	⬆️
src/heartbeat.js	62.33% <0.00%> (+1.84%)	⬆️

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