last_value function for alarm sub-expressions

- Add a new function, last_value, to alarm sub-expressions that use the value of the last metric that was sent when evaluating alarms.
- In the Threshold Engine, immediately transition to the ALARM'd or OK state, based on the value of the last metric that exceeds the threshold or is less than the threshold, respectively.
    - Note, if the metric is not continually sent, the alarm will continue to use the value of the last metric sent, even across alarm evaluation periods. In other words, the alarm will not automatically transition to the UNDETERMINED or OK state, even if metrics are no longer sent.
- Modifications:
    - Add state column to the sub_alarm table/schema.
        - Currently, the state of alarm subs-expressions are not saved, only the state of alarms are saved.
    - Threshold Engine updates state in the sub_alarm table when state transitions occur.
    - Threshold Engine initializes sub-alarms states on start-up.
    - Add support in the Threshold Engine for the last_value function.
    - Add support in the Python and Java APIs for the last_value function in the alarm sub-expression parsing.
    - Add support in the python-monascaclient for the last_value function.
- No new APIs/resources are required to be added.
- Validate performance.