Avoid a tree of merge objects, which can result in
what I suspect is n^2 calls to Seek when using Without.
With 100k metrics, and a regex of ^$ in BenchmarkHeadPostingForMatchers:
Before:
BenchmarkHeadPostingForMatchers-8 1 51633185216 ns/op 29745528 B/op 200357 allocs/op
After:
BenchmarkHeadPostingForMatchers-8 10 108924996 ns/op 25715025 B/op 101748 allocs/op
Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>
This saves memory, about a quarter of the size of the postings map
itself with high-cardinality labels (not including the post ids).
Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>
