PBFT and forwarding client certificates

[ashamis]

• Do not forward the request signatures unless required - with PBFT
• Forward the signatures for the members frontend

[jumaffre]

As discussed, there's a more general bug here since #962. We should no longer forward the caller certificate uniquely based on which frontend we are executing on but based on whether the handler required client identity too. I'll create an issue for this but it shouldn't block this PR.

[codecov-io]

Codecov Report

Merging #971 into master will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #971      +/-   ##
==========================================
- Coverage   68.07%   68.05%   -0.02%     
==========================================
  Files         103      103              
  Lines        8177     8175       -2     
==========================================
- Hits         5566     5563       -3     
- Misses       2611     2612       +1     

Flag	Coverage Δ
#unit_BFT	?
#unit_CFT	68.05% <0.00%> (?)

Impacted Files	Coverage Δ
src/node/rpc/frontend.h	64.23% <0.00%> (ø)
src/node/rpc/memberfrontend.h	67.46% <ø> (-0.12%)	⬇️
src/ds/ringbuffer.h	92.39% <0.00%> (-0.54%)	⬇️

[ghost]

less_msg@6263 aka 20200323.45 vs master ewma over 30 builds from 5953 to 6261