Added the `META name value` keyword to the SETHOOK command.
Allows for adding metadata to a webhook. For example:
SETHOOK myhook http://endpoint/ META m1 12 META m2 13 NEARBY ...
Would result in notification that contain the "meta" element, which is
represented like:
"meta":{"m1":"12","m2":"13"}
Thanks for the suggestion @amorskoy
closed#105
This addresses an issue #73 that @huangpeizhi discovered while using
Tile38 in production where AOFSHRINK sometimes corrupts the database
causing the server to not start the next time.