Some RPCs are GET-only

[eddyashton]

Resolves #633.

This PR:

• Adds support for restricting which verbs are supported for a specific RPC (install(...).set_http_get_only(), or more generally .restrict_allowed_verbs({HTTP_DELETE, HTTP_GET,...})
• Sets some built-in RPCs to be GET-only (getCommit, listMethods, etc). Mostly things that are Read and have simple params types
• Modifies the Python infra to take a specific verb, in particular using c.get rather than c.rpc
• Adds a url-decoder/unescaper. For GETs we need to put params in the query, not the body. Some of these queries need escaping (whenever we have a json-string value, its surrounding quotes need to be escaped). I spent a long time trying to convince Python-requests to not escape the query, but all the workarounds are just arounds (they don't work), so I bit the bullet and wrote a quick escaper on the C++ side
• Related tidying (removing the unneeded client.do in Python, removing empty params so we have actually empty bodies rather than JSON nulls)

NB: I'm using "verb" to describe GET, POST etc. These are generally referred to as methods, but we already have a lot of code using method to describe the RPC-identifier. This is a bit clumsy - maybe we should do a renaming pass to make the old method a resource, and then we can use method instead of verb? Opinions welcome.

[jumaffre]

Looks like there are still some issues with the CI but the changes look good!

[ghost]

verb_selection@6187 aka 20200323.6 vs master ewma over 30 builds from 5897 to 6181

[codecov-io]

Codecov Report

Merging #966 into master will increase coverage by 0.22%.
The diff coverage is 95.4%.

@@            Coverage Diff             @@
##           master     #966      +/-   ##
==========================================
+ Coverage   68.06%   68.28%   +0.22%     
==========================================
  Files         103      103              
  Lines        8177     8234      +57     
==========================================
+ Hits         5565     5622      +57     
  Misses       2612     2612

Flag	Coverage Δ
#unit_BFT	68.28% <95.4%> (+0.22%)	⬆️
#unit_CFT	68.28% <95.4%> (+0.22%)	⬆️

Impacted Files	Coverage Δ
src/http/http_rpc_context.h	39.39% <0%> (+0.46%)	⬆️
src/node/rpc/memberfrontend.h	67.4% <100%> (-0.18%)	⬇️
src/node/rpc/frontend.h	66.09% <100%> (+1.86%)	⬆️
src/node/rpc/handlerregistry.h	81.82% <100%> (+2.87%)	⬆️
src/node/rpc/commonhandlerregistry.h	28.57% <100%> (+2.76%)	⬆️
src/node/rpc/nodefrontend.h	79.01% <100%> (+0.35%)	⬆️
src/node/rpc/jsonhandler.h	83.67% <100%> (+0.52%)	⬆️
src/http/http_parser.h	88.57% <95.83%> (+1.39%)	⬆️
src/tls/keypair.h	74.69% <0%> (+0.31%)	⬆️