certificates: coalesce concurrent validations of the same host

[danzh2010]

#2525 made cronet and c++ engines be able to use platform provided APIs to asynchronously verify certificates. It is done by creating stand-alone threads to call into platform APIs when each validation is requested. Doing so ensures Envoy Mobile's network thread won't be blocked by these platform APIs calls, but one thread for one validation might be expensive for both memory and CPU. We want to improve that by limiting the number of validation threads and coalesce concurrent validations of the same host.