colblk: add SeekUnsetBitGE/LE

[RaduBerinde]

colblk: move out nextInBit/prevInBit functions

This change uninlines these functions and adds a test for them.

colblk: rename Successor/Predecessor to SeekSetBitGE/SeekSetBitLE

The previous names were misleading since we can return the same
index.

colblk: add SeekUnsetBitGE/LE

These variants will be useful for skipping blocks of set bits, for
example to skip obsolete keys.

[cockroach-teamcity]

This change is 

[jbowens]

Reviewed 2 of 2 files at r1, 6 of 6 files at r2, 3 of 3 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @RaduBerinde)

---

sstable/colblk/bitmap_test.go line 90 at r1 (raw file):

	for _, w := range words {
		// Check that we can reconstruct the word if we jump from set bit to set
		// bit.

nice test

[RaduBerinde]

TFTR!