p2p dht: more tests and minor fixes

[algorandskiy]

1. Fixed backoff init parameter base - setting it to 1 causes backoff logic to fail on the first backoff.
2. Speed up TestCapabilities_Varying and test partial DHT bootstrap.

[codecov]

Codecov Report

Merging #5827 (42b4c3b) into feature/p2p (4349e79) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           feature/p2p    #5827      +/-   ##
===============================================
+ Coverage        55.65%   55.67%   +0.02%     
===============================================
  Files              477      477              
  Lines            66992    66993       +1     
===============================================
+ Hits             37282    37297      +15     
+ Misses           27184    27170      -14     
  Partials          2526     2526              

Files	Coverage Δ
network/p2p/dht/dht.go	100.00% <100.00%> (ø)

... and 17 files with indirect coverage changes

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