Set random percentages through admin wfe at runtime

[mbaynton]

Changing these percentages can simplify creating test suites where certain
tests exercise authorization reuse or nonce error handling.

[squizzling]

Although hard to trigger, these create data races:

==================
WARNING: DATA RACE
Read at 0x00c0001161f8 by goroutine 66:
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).verifyJWS()
      /tank/tank/src/pebble/wfe/wfe.go:934 +0xd85
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).verifyPOST()
      /tank/tank/src/pebble/wfe/wfe.go:876 +0x395
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).Authz()
      /tank/tank/src/pebble/wfe/wfe.go:2040 +0x91
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).Authz-fm()
      /tank/tank/src/pebble/wfe/wfe.go:2033 +0x84
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).HandleFunc.func1()
      /tank/tank/src/pebble/wfe/wfe.go:283 +0x9b8
  github.com/letsencrypt/pebble/wfe.wfeHandlerFunc.ServeHTTP()
      /tank/tank/src/pebble/wfe/wfe.go:134 +0x83
  github.com/letsencrypt/pebble/wfe.(*topHandler).ServeHTTP()
      /tank/tank/src/pebble/wfe/wfe.go:146 +0x6e
  net/http.StripPrefix.func1()
      /usr/local/go/1.16.0/src/net/http/server.go:2112 +0x535
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/go/1.16.0/src/net/http/server.go:2069 +0x51
  net/http.(*ServeMux).ServeHTTP()
      /usr/local/go/1.16.0/src/net/http/server.go:2448 +0xaf
  net/http.serverHandler.ServeHTTP()
      /usr/local/go/1.16.0/src/net/http/server.go:2887 +0xca
  net/http.(*conn).serve()
      /usr/local/go/1.16.0/src/net/http/server.go:1952 +0x87d

Previous write at 0x00c0001161f8 by goroutine 76:
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).updateRuntimeConfig()
      /tank/tank/src/pebble/wfe/wfe.go:519 +0x3a4
  github.com/letsencrypt/pebble/wfe.(*WebFrontEndImpl).updateRuntimeConfig-fm()
      /tank/tank/src/pebble/wfe/wfe.go:489 +0x84
  github.com/letsencrypt/pebble/wfe.wfeHandlerFunc.ServeHTTP()
      /tank/tank/src/pebble/wfe/wfe.go:134 +0x83
  net/http.StripPrefix.func1()
      /usr/local/go/1.16.0/src/net/http/server.go:2112 +0x535
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/go/1.16.0/src/net/http/server.go:2069 +0x51
  net/http.(*ServeMux).ServeHTTP()
      /usr/local/go/1.16.0/src/net/http/server.go:2448 +0xaf
  net/http.serverHandler.ServeHTTP()
      /usr/local/go/1.16.0/src/net/http/server.go:2887 +0xca
  net/http.(*conn).serve()
      /usr/local/go/1.16.0/src/net/http/server.go:1952 +0x87d

Goroutine 66 (running) created at:
  net/http.(*Server).Serve()
      /usr/local/go/1.16.0/src/net/http/server.go:3013 +0x644
  net/http.(*Server).ServeTLS()
      /usr/local/go/1.16.0/src/net/http/server.go:3053 +0x433
  net/http.(*Server).ListenAndServeTLS()
      /usr/local/go/1.16.0/src/net/http/server.go:3208 +0x192
  net/http.ListenAndServeTLS()
      /usr/local/go/1.16.0/src/net/http/server.go:3174 +0x126e
  main.main()
      /tank/tank/src/pebble/cmd/pebble/main.go:101 +0x126f

Goroutine 76 (finished) created at:
  net/http.(*Server).Serve()
      /usr/local/go/1.16.0/src/net/http/server.go:3013 +0x644
  net/http.(*Server).ServeTLS()
      /usr/local/go/1.16.0/src/net/http/server.go:3053 +0x433
  net/http.(*Server).ListenAndServeTLS()
      /usr/local/go/1.16.0/src/net/http/server.go:3208 +0x192
  net/http.ListenAndServeTLS()
      /usr/local/go/1.16.0/src/net/http/server.go:3174 +0x1e4
  main.main.func1()
      /tank/tank/src/pebble/cmd/pebble/main.go:80 +0x1e5
==================

[mbaynton]

@squizzling thanks for the review, and props for triggering the race detector especially sans applicable tests. I think my shields were probably lowered in this respect because I anticipated use cases where adjustment of the percentages was de-facto sequential before usage of Pebble that would generate reads, but just eliminating the possibility is definitely better.

The data was conveniently of a type that can be accessed using processor-level atomics, and since there was not any traditional locking already going on in wfe I just fixed it that way.