Redis output to store last values

[streamnsight]

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This PR augments the original logstash-redis-output plugin to store the last value(s) of each key in the event object.

The original redis output stores every event in a LIST, which just grows undefinitely unless trimmed by some other consumer.

This PR adds 2 modes:

• last_n_values will store the last n values of each key in the event object into a LIST (LPUSH and TRIM) with an EXPIRE value if defined. stack_size defines how many values are kept.
• last_value will store the last value of each key in the event object with SET with an EXPIRE value if defined.

In both cases the keys are stores under the top level key defined by key, into a tree structure matching the object.

Each write is done in a MULTI transaction so as to be sure to store all key/values at once.

Note: Since 5.alpha is alpha and not quite out yet, the PR is for Logstash 2.3.x, so the version number and dependencies are matching this version.

Let me know if this needs more attention.