rpc/client: split out client packages

[erikgrinaker]

Fixes #4626:

• Split rpc/client implementations into rpc/client/http and rpc/client/local, and rename constructors.
• Split lite2 tests into separate lite_test package.
  • Embed SignStatusClient in http.http struct, so tests can convert it.

---

For contributor use:

• Wrote tests
• Updated CHANGELOG_PENDING.md
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments
• Re-reviewed Files changed in the Github PR explorer

[codecov-io]

Codecov Report

Merging #4628 into master will increase coverage by 0.89%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #4628      +/-   ##
==========================================
+ Coverage   65.43%   66.32%   +0.89%     
==========================================
  Files         229      220       -9     
  Lines       20444    19608     -836     
==========================================
- Hits        13377    13005     -372     
+ Misses       6004     5587     -417     
+ Partials     1063     1016      -47     

Impacted Files	Coverage Δ
cmd/tendermint/commands/lite.go	24.65% <0.00%> (ø)
lite2/provider/http/http.go	28.57% <57.14%> (ø)
lite/client/provider.go	38.09% <100.00%> (ø)
p2p/pex/errors.go	11.11% <0.00%> (-11.12%)	⬇️
p2p/metrics.go	95.23% <0.00%> (-4.77%)	⬇️
p2p/pex/pex_reactor.go	82.58% <0.00%> (-1.69%)	⬇️
consensus/replay.go	71.76% <0.00%> (-0.79%)	⬇️
p2p/pex/addrbook.go	70.16% <0.00%> (-0.47%)	⬇️
rpc/lib/server/http_uri_handler.go
rpc/core/types/codec.go
... and 21 more

[cmwaters]

LGTM 👍

[tac0turtle]

we should add a sentence or two to upgrading.md about this

[erikgrinaker]

Good call