Added channel caching by host string

src/objective-c/GRPCClient/private/GRPCChannel.m

@@ -40,9 +40,19 @@
 
 @implementation GRPCChannel
 
+// TODO(mlumish): Investigate whether a cache with strong links is a good idea
+static NSMutableDictionary *channelCache;
+
 + (instancetype)channelToHost:(NSString *)host {
-  // TODO(jcanizales): Reuse channels.
-  return [[self alloc] initWithHost:host];
+  if (channelCache == nil) {
+    channelCache = [NSMutableDictionary dictionary];
+  }
+  GRPCChannel *channel = channelCache[host];
+  if (channel == nil) {
+    channel = [[self alloc] initWithHost:host];
+    channelCache[host] = channel;
+  }
+  return channel;
 }
 
 - (instancetype)init {