ANN: Optimize host-side refine

[achirkin]

Prior to this change, raft's host-side implementation of raft::neighbors::refine operation uses non-optimal OpenMP thread config by default, spawning as many threads as there are available cores, even if only one thread is used (per-query parallelism with batch size one).
This change fixes that and adds a few optimizations alongside:

• Use the number of threads = min(cores, queries)
• Use templates to push the metric-type condition outside the distance computation loop (this should also make it easier to implement new metrics later)
• Force tree-optimize compilation flag in the hopes compiles does the vectorization
• Split out the host implementation in separate files to be able to compile it without nvcc.

[tfeher]

Thanks Artem for the PR, it looks good to me (apart from the issue with the unnecessary extra compile flag)!

[achirkin]

On the performance note: the new code gives some ridiculous speed up of up to x500-x1000 in the case of small batches (n_query = 1), I'm not sure why. Also one cannot rely on the perf reporting in the current prims bench, because it uses cuda events to measure the time, while the host-only refine obviously does not use cuda streams (#1653). I will bring up the necessary updates to the benchmarks in a follow-up PR.

[cjnolet]

/merge