Simplify signature of handler install

[eddyashton]

Resolves #914.

We have several parameters to tweak the behaviour of installed RPC handlers, and we're likely to add more. Rather than needing to specify all of these in the call to install (with signficant overload complications), they should be set manually on the few handlers they actually affect.

This PR lets you set those parameters in 2 ways. Manually:

auto& foo_handler = install("FOO", foo_func, Read);
foo_handler.execute_locally = true;
foo_handler.params_schema = ...;

Or with chaining methods:

install("FOO", foo_func, Read)
  .set_execute_locally(true)
  .set_params_schema(...);

I prefer the second, but I think its worth keeping both in general.

[codecov-io]

Codecov Report

Merging #960 into master will decrease coverage by 0.05%.
The diff coverage is 95.29%.

@@            Coverage Diff             @@
##           master     #960      +/-   ##
==========================================
- Coverage   68.13%   68.09%   -0.05%     
==========================================
  Files         103      103              
  Lines        8146     8147       +1     
==========================================
- Hits         5550     5547       -3     
- Misses       2596     2600       +4

Flag	Coverage Δ
#unit_BFT	68.09% <95.29%> (-0.03%)	⬇️
#unit_CFT	68.07% <95.29%> (-0.05%)	⬇️

Impacted Files	Coverage Δ
src/node/rpc/memberfrontend.h	67.41% <100%> (+0.43%)	⬆️
src/node/rpc/commonhandlerregistry.h	25.81% <100%> (-0.48%)	⬇️
src/node/rpc/userfrontend.h	100% <100%> (ø)	⬆️
src/node/rpc/frontend.h	64.77% <100%> (ø)	⬆️
src/node/rpc/nodefrontend.h	78.65% <100%> (ø)	⬆️
src/node/rpc/handlerregistry.h	82.98% <84%> (-9.33%)	⬇️

[ghost]

simple_install_signature@6059 aka 20200317.11 vs master ewma over 30 builds from 5763 to 6053

[achamayou]

@eddyashton this is great! I think we should have the accessors only, and make sure they're exposed in doxygen/sphinx.