ConnectionManager HTTP/2 updates (#360)

- Fix a bug about goaway received. Not release a connection that's in used.
- Don't vend connection to user until the settings done. To help user knows that the connection is really ready to use.

TODO: 
- Does it worth to put tests about http2 stuff here as we also test the stream manager stuff for http2 as the user of connection manager? I think we can just add todo here...

Author: TingDaoK

include/aws/http/connection_manager.h

@@ -121,6 +121,8 @@ struct aws_http_connection_manager *aws_http_connection_manager_new(
  * Requests a connection from the manager.  The requester is notified of
  * an acquired connection (or failure to acquire) via the supplied callback.
  *
+ * For HTTP/2 connections, the callback will not fire until the server's settings have been received.
+ *
  * Once a connection has been successfully acquired from the manager it
  * must be released back (via aws_http_connection_manager_release_connection)
  * at some point.  Failure to do so will cause a resource leak.

include/aws/http/private/connection_manager_system_vtable.h

@@ -19,6 +19,8 @@ typedef bool(aws_http_connection_is_connection_available_fn)(const struct aws_ht
 typedef bool(aws_http_connection_manager_is_callers_thread_fn)(struct aws_channel *channel);
 typedef struct aws_channel *(aws_http_connection_manager_connection_get_channel_fn)(
     struct aws_http_connection *connection);
+typedef enum aws_http_version(aws_http_connection_manager_connection_get_version_fn)(
+    const struct aws_http_connection *connection);
 
 struct aws_http_connection_manager_system_vtable {
     /*
@@ -31,6 +33,7 @@ struct aws_http_connection_manager_system_vtable {
     aws_io_clock_fn *get_monotonic_time;
     aws_http_connection_manager_is_callers_thread_fn *is_callers_thread;
     aws_http_connection_manager_connection_get_channel_fn *connection_get_channel;
+    aws_http_connection_manager_connection_get_version_fn *connection_get_version;
 };
 
 AWS_HTTP_API