Change the log_hash to use string keys, not symbol keys.

[suprememoocow]

Disclaimer: My ruby skills are not strong!

When attempting to match this source to the GELF output plugin, out_gelfplugin is failing with a TypeError related to the hash keys being Symbols, not strings. This PR changes the hash structure as recommended in the fluentd plugin, which fixes output to the GELF plugin.

The timestamp of the slow operation from Redis is now used as the log timestamp, instead of the time that the slow is read. This allows easier correlation with associated events (otherwise events will be reported as having happened up to interval seconds after the log actually occurred)

I've also made the logsize optional, as the Redis SLOWLOG GET command does not require a number of records. If no number is specified, all entries are returned.