[apps] adding backoff to state saving in apps functions config

[ryandeivert]

to: @chunyong-lin
cc: @airbnb/streamalert-maintainers
size: small
resolves #682

Background

See #682, gist: rapid state saving can cause issues with SSM and will produce a TooManyUpdates error

Changes

• Adding backoff to the _save_state function that will backoff on ClientError exceptions.
• The saving will be attempted 5 times and will then raise an exception.
• Adding backoff to app integration packaging requirements.

Testing

Updating unit tests to patch new MAX_SAVE_TRIES class property, but no other test changes were required for this.

[coveralls]

Coverage increased (+0.005%) to 95.894% when pulling 3051645 on ryandeivert-backoff-app-ssm into 6931ee1 on master.

[chunyong-lin]

Thanks for the change!