Fix typo in README.md
Create get_connection method on Pool to simplify daemon code.
Merge branch 'refactor_connection_pool'

* refactor_connection_pool:
  Create get_connection method on Pool to simplify daemon code.
  Fix typo in README.md
Use fibers to untangle the callback spagetti a little.
Fix possible race condition when deleting connections
Fix info log message
Track and consider APN invalid token errors.

Added some hash structures to ConnectionPool to keep track of invalid tokens.

Added hash structure on APNConnection to keep track of message token/id
mappings.

Register invalid tokens received by APN.

Check for token validity when building notifications and ignore all invalid
tokens.
Remove duplicate method
Add connection timeout to apn_connections
Reimplement APN feedback service.
Merge branch 'reimplement_feedback' into use_fibers

* reimplement_feedback:
  Reimplement APN feedback service.
  Add connection timeout to apn_connections
Merge branch 'use_fibers'

* use_fibers:
  Reimplement APN feedback service.
  Add connection timeout to apn_connections
  Remove duplicate method
  Track and consider APN invalid token errors.
  Fix info log message
  Fix possible race condition when deleting connections
  Use fibers to untangle the callback spagetti a little.