Previously, a given binary would have three TLS config fields (CA cert, cert,
key) for its gRPC server, plus each of its configured gRPC clients. In typical
use, we expect all three of those to be the same across both servers and clients
within a given binary.
This change reuses the TLSConfig type already defined for use with AMQP, adds a
Load() convenience function that turns it into a *tls.Config, and configures it
for use with all of the binaries. This should make configuration easier and more
robust, since it more closely matches usage.
This change preserves temporary backwards-compatibility for the
ocsp-updater->publisher RPCs, since those are the only instances of gRPC
currently enabled in production.
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