add FixedBitSet.nextUnsetBit and use it in MergingHnswGraphBuilder

[iprithv]

there was a TODO in MergingHnswGraphBuilder.build() where we were scanning all nodes even if most were already initialized, this change adds nextUnsetBit and uses that instead, so we only go over nodes that are not set, instead of checking every node, we just jump to the next unset one.

so earlier it goes through all nodes and check each one, now jump directly to next unset bit.
this is faster when most bits are already set, which is common during merges

benchmark (bitset iteration)

size	set %	scan loop (us)	nextUnsetBit (us)	speedup
1,000	0%	8.76	11.89	slower
1,000	75%	2.27	1.25	~1.8x
1,000	90%	2.30	0.54	~4.2x
1,000	99%	2.24	0.06	~35x
100,000	75%	217	82.3	~2.6x
100,000	90%	218	33.4	~6.5x
100,000	99%	215	4.69	~45x
1,000,000	99%	2157	43.3	~49x

luceneutil (real run)

	baseline	candidate	result
force merge	220.11s	218.68s	same
index time	224.73s	229.25s	same
recall	0.988	0.988	same

[iverase]

see #16072

[iprithv]

thanks @iverase for pointing out, will close this. As that was merged just now i missed it :)