Memory registry from #801 with additional tweaks (#951)

* PoC: memory registry using maps instead of slice madness

* Updated proto and handlers. Fixed tests across codebase.

* Implemented ttl pruning for memory registry

* Added extensive memory registry tests

* Squased a bunch of bugs

* Proto indent; memory.Registry.String() returns "memory"

* Write a test to prove memory registry TTLs are busted

Signed-off-by: Erik Hollensbe <github@hollensbe.org>

* Additional memory testing and fixups:

* DefaultTTL removed
* When TTL == 0, it is automatically removed from expiry conditions
* Additional improvements to new tests

Signed-off-by: Erik Hollensbe <github@hollensbe.org>

client/selector/default_test.go

@@ -9,9 +9,7 @@ import (
 func TestRegistrySelector(t *testing.T) {
 	counts := map[string]int{}
 
-	r := memory.NewRegistry()
-	rg := r.(*memory.Registry)
-	rg.Services = testData
+	r := memory.NewRegistry(memory.Services(testData))
 	cache := NewSelector(Registry(r))
 
 	next, err := cache.Select("foo")