Show the number of UIDs assigned

[tsuna]

Add a command (under tsdb uid) and possibly an HTTP end point to report the number of UIDs currently allocated for metric names, tag names, and tag values.

[tsuna]

Workaround that can be used in the mean time:

$ ./build/tsdb uid grep metrics . | wc -l
663

and subtract one from the result printed (because there is always an extra row printed that isn't a metric). To get the count of tag names, specify tagv instead of metrics. To get the count of tag values, just specify tagk instead.

[manolama]

Completed in "next" branch with Isavin's excellent work.