Serve HTTP2 health check

[gdbelvin]

Prioritize HTTP2 over HTTP1

Revert #1435 and use http.ServeHTTP rather than cmux.

• cmux prevents http.Serve from serving HTTP2
• http.Serve will only serve HTTP2 over TLS
  • http.Serve uses a type assertion to validate this
  • cmux interrupts this because it wraps the connection.
  • even if the connection did come from a TLS listener

The GCE ingress load ballancer requires services to serve
an HTTP 200 OK at / on the same protocol that the server uses.
To support GRPC, this means using HTTP2 throuought.
HTTP2 also requires TLS, both by golang and GCE ingress.

[codecov]

Codecov Report

Merging #1442 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1442      +/-   ##
==========================================
- Coverage   66.56%   66.31%   -0.25%     
==========================================
  Files          54       54              
  Lines        4026     4026              
==========================================
- Hits         2680     2670      -10     
- Misses        955      962       +7     
- Partials      391      394       +3

Impacted Files	Coverage Δ
core/client/mutations.go	83.78% <0%> (-5.41%)	⬇️
core/keyserver/revisions.go	62.58% <0%> (-2.05%)	⬇️
impl/sql/directory/storage.go	67.66% <0%> (-1.51%)	⬇️
core/sequencer/server.go	72.96% <0%> (-0.98%)	⬇️

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