connect: simplify the compiled discovery chain data structures

This should make them better for sending over RPC or the API.

Instead of a chain implemented explicitly like a linked list (nodes
holding pointers to other nodes) instead switch to a flat map of named
nodes with nodes linking other other nodes by name. The shipped
structure is just a map and a string to indicate which key to start
from.

Other changes:

* inline the compiler option InferDefaults as true

* introduce compiled target config to avoid needing to send back
  additional maps of Resolvers; future target-specific compiled state
  can go here

* move compiled MeshGateway out of the Resolver and into the
  TargetConfig where it makes more sense.

Author: rboyer

agent/consul/config_endpoint.go

@@ -361,7 +361,6 @@ func (c *ConfigEntry) ReadDiscoveryChain(args *structs.DiscoveryChainRequest, re
 				OverrideMeshGateway:    args.OverrideMeshGateway,
 				OverrideProtocol:       args.OverrideProtocol,
 				OverrideConnectTimeout: args.OverrideConnectTimeout,
-				InferDefaults:          true,
 				Entries:                entries,
 			})
 			if err != nil {