[k-NN] Avoid additional copy to stream during binary doc values deserialization

[jmazanec15]

Description

Related #1709

During deserialization, we have an intermediate step where we copy a bytesref to a bytesstream. This can be removed as it adds some overhead and does add any benefit. With #1087, this will be no longer necessary. But until then, it could add some benefit.

From the experiments in #1709 , the diff with and without the optimization is:

Results - w/o change

Run #	p50 latency (ms)	p90 latency (ms)	p99 latency (ms)	Recall
1	674	684	692	0.99998
2	674	684	692	0.99998

Results - w/ change

Run #	p50 latency (ms)	p90 latency (ms)	p99 latency (ms)	Recall
1	568	584	594	0.99998
2	568	584	596	0.99998

PoC commit: jmazanec15@7984618.

[navneet1v]

This intermediate step of copy byteref to bytestream is also present when we read vectors during merge. Hence marking this issue as indexing-improvements too.

[navneet1v]

@jmazanec15 can we close this GH issue as the feature is added in k-NN plugin,