Create an internal stat for dropped streams

[at055612]

Relates to #1965

Would be useful for stroom to record when streams are dropped, e.g. if a system is sending many empty files.

The stat should prob have tags: feed system env some sort of enum to describe the reason for the drop, e.g. empty, policy rule, etc.

Will need a new stat dat src creating in the internal stats pack in stroom-content (sql + stroom-stats), with a new release of that.