iproto: add watchers support

Part of #6257

@TarantoolBot document
Title: Document IPROTO watchers

There are three new commands to support asynchronous server->client
notifications signaled with `box.broadcast()`:

 - `IPROTO_WATCH` (code 74). Registers a new watcher for the given
   notification key or acknowledges a notification if a watcher is
   already registered. The key name is passed in `IPROTO_EVENT_KEY`
   (code 0x56). The watcher will be notified unconditionally after
   registration and then every time the key is updated with
   `box.broadcast()` provided the last notification was acknowledged.
   The server doesn't reply to the request unless it fails to parse
   the packet.

 - `IPROTO_UNWATCH` (code 75). Unregisters a watcher registered for the
   given notification key. The key name is passed in `IPROTO_EVENT_KEY`
   (code 0x56). A server doesn't reply to the request unless it fails to
   parse the packet.

 - `IPROTO_EVENT` (code 76). Sent by the server to notify a client
   about a key update. The key name is passed in `IPROTO_EVENT_KEY`
   (code 0x56). The key data (optional) is passed in `IPROTO_EVENT_DATA`
   (code 0x57).

When a connection is closed, all watchers registered for it are
unregistered.

Servers that support the new feature set the `IPROTO_FEATURE_WATCHERS`
feature bit (bit 3) in reply to the `IPROTO_ID` command.