refactor: reuse the slice in MemPostings.Delete()

[YaoZengzeng]

Signed-off-by: YaoZengzeng [email protected]

[bwplotka]

I think I like this.

The

	               found := false
			for _, id := range p.m[n][l] {
				if _, ok := deleted[id]; ok {
					found = true
					break
				}
			}
			if !found {
				p.mtx.Unlock()
				continue
			}

Was a quick path to not allocate new slice. Since we reuse slice we don't need to worry about that.
We can drop this since not matter if found or not we would do full loop over p.m[n][l]. In fact in old code we do it twice.

LGTM from my side, thank you for that!
Wonder if we can do some benchmarks for this to actually see the difference? In theory it should be faster.

[krasi-georgiev]

Automated tsdb bench tests triggered from a comment coming soon(probably next week) so this would a good test for it.