use structured debug logging, and wrap JSON payloads in repr()

[meejah]

This is a fix for #599 which also fixes the debug-logging from frames to use structured-style logging -- since we already special-cased JSON payloads (opcode == 1) this just wraps those in a repr() so they're properly escaped (and also easy to copy/paste now).

[codecov-io]

Current coverage is 57.34%

Merging #606 into master will increase coverage by +0.01% as of 01f332d

@@            master    #606   diff @@
======================================
  Files           61      61       
  Stmts        10240   10238     -2
  Branches      1664    1664       
  Methods          0       0       
======================================
  Hit           5871    5871       
  Partial        556     556       
+ Missed        3813    3811     -2

Review entire Coverage Diff as of 01f332d

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