Fix subscription packet callbacks not executing within processPacketsUntil()

[brentru]

Callback functions within subscription packets are currently not processed if processPacketsUntil() receives a subscription packet. While processPacketsUntil() does call handleSubscriptionPacket(), handleSubscriptionPacket() fills the Adafruit_Subscription object but does not call the callbacks as mentioned in its docstring ("// Handle any data coming in for subscriptions and fires them off to the appropriate callback") nor does it mark the subscription message as "read" (implemented in https://github.com/adafruit/Adafruit_MQTT_Library/pull/195).

This causes any subscription packet picked up during processPacketsUntil()'s period (called by a ping() or publish()) to never execute its callback method.

This pull request:

• Adds a new method, processSubscriptionPacket which executes a subscription packet's associated callback and marks the message as "read"
• Refactors processPackets and processPacketsUntil to use processSubscriptionPacket

This was tested against the Adafruit IO WipperSnapper library's synchronization function and was OK for: 5x servos re-syncing, 2x DS18x components syncing, 2x i2c devices + servo + 2x dsx + 1x pin component syncing

ping @ladyada for review