feat: make log as protected

[twoeths]

Motivation
Lodestar wants to incorporate gossipsub log to its own log files

Description

• According to https://github.com/debug-js/debug#output-streams we should be able to configure per-namespace by overriding the log method, however when I tried in lodestar it does not work and we have to use the same log instance
• Change log from private to protected so that child class can access and redirect to its own log

Closes #406

[what-the-diff]

• Add a new property log to the class.
• Make it protected so child classes can override and redirect logs to their own loggers if needed.
• Remove unused variable declaration of directPeerInitial in constructor, since we are not using this anymore after refactoring codebase for v1 protocol implementation (https://github.com/libp2p/js-libp2p-gossipsub/pull/534).

[codecov-commenter]

Codecov Report

Base: 83.31% // Head: 83.29% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (1903dc3) compared to base (1e1cf5a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
- Coverage   83.31%   83.29%   -0.02%     
==========================================
  Files          48       48              
  Lines       11800    11804       +4     
  Branches     1271     1270       -1     
==========================================
+ Hits         9831     9832       +1     
- Misses       1969     1972       +3     

Impacted Files	Coverage Δ
src/index.ts	70.40% <100.00%> (-0.07%)	⬇️

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