Add 'max_connections' as an understood config field to gcmpushkin

[anoadragon453]

We were getting the following harmless error in our logs:

[...] sygnal.gcmpushkin 109 - WARNING - The following configuration fields are not understood: {'max_connections'}

However this option is documented in the sample config:

sygnal/sygnal.yaml.sample

Lines 245 to 247 in 08318c8

	# # This is the maximum number of connections to GCM servers at any one time
	# # the default is 20.
	# #max_connections: 20

and used in the codebase:

sygnal/sygnal/gcmpushkin.py

Lines 114 to 116 in 0672a84

	self.max_connections = self.get_config(
	"max_connections", DEFAULT_MAX_CONNECTIONS
	)

So it seems like we just forgot to add it to this list while adding the option 🙂

(If you're wondering why inflight_request_limit isn't in this dict either, that's because it's shared across both APNs and GCM pushkin types :)