This commit adds configuration revision, which is considered opaque
string, and changes after reloading configuration with SIGHUP. This
revision is returned as a response to configrevision API endpoint.
This allows external application to know whether nghttpx has finished
reloading new configuration or not. Note that this revision does not
change on backendconfig API calls.
For those connections via frontend with api parameter, they use solely
APIDownstreamConnection.
In this commit, APIDownstreamConnection just consumes all request
body, and do nothing. The next few commits implements our first API
endpoint: /v1/api/dynamicconfig.