Add docs outlining our performance test, tx time plotting script

[eddyashton]

I've moved a script from old tests to samples, cleaned it up, and added a docs page describing our micro-benchmark and e2e perf tests.

Open to rewrites on this: should we walk through extending perf_client for user apps? Should we show more comparisons of bad performance (memory blowup, busy machine)?

[cimetrics]

[codecov-io]

Codecov Report

Merging #461 into master will decrease coverage by 0.21%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage    82.7%   82.49%   -0.21%     
==========================================
  Files         118      118              
  Lines        9519     9519              
==========================================
- Hits         7872     7852      -20     
- Misses       1647     1667      +20

Flag	Coverage Δ
#e2e_BFT	53.2% <ø> (-0.22%)	:arrow_down:
#e2e_CFT	70.36% <ø> (-1.42%)	:arrow_down:
#unit_BFT	65.09% <ø> (-0.02%)	:arrow_down:
#unit_CFT	71.32% <ø> (+0.01%)	:arrow_up:

Impacted Files	Coverage Δ
src/node/rpc/memberfrontend.h	79.54% <0%> (-4.03%)	:arrow_down:
src/apps/luageneric/luageneric.cpp	81.58% <0%> (-3.95%)	:arrow_down:
src/luainterp/luajson.h	92.11% <0%> (-1.75%)	:arrow_down:
src/ds/ringbuffer.h	92.35% <0%> (-0.55%)	:arrow_down:

[cimetrics]

[eddyashton]

The csv files for the 200k transactions are pretty big (about as many lines as our 3rdparty/ directory for each file). Do we need to include them at all or is it easy enough to reproduce them?

I was keeping them around as good exemplars while I fiddled with the plotting, but I think we don't need the csv files now.

[cimetrics]

[cimetrics]

[cimetrics]

[cimetrics]

[cimetrics]

[cimetrics]

[achamayou]

This somewhat unrelated, but we should add units in the names of the metrics we publish.

[cimetrics]