Direct mapped visited set imp

[TamimEhsan]

This PR adds a new epoch based visited set implementation and uses compile time abstraction for the visited set. The default implementation is still the HashBasedBooleanSet and the implementation stays the same exactly. So, no regression introduced.

Why the new Epoch Based Set

The hash based set due to its use of unordered set needs dynamic memory allocation and deallocation. So, that contributes to the whole QPS. For higher topk or EF this impacts the overall runtime. So, an alternative implementation is added. The choice is still hardcoded in visited_set.hpp. That's a task for the future.

Performance

Dataset: GIST10M, Topk = 10, quantization = 9 bit

EF	Recall	HashBased QPS (Old)	EpochBased QPS (New)	Multiplier (New/Old)
10	0.4913	12735.1	12259.8	0.96x
20	0.6230	9776.5	9515.3	0.97x
40	0.7564	6387.2	6603.5	1.03x
80	0.8495	4469.7	4559.3	1.02x
120	0.8920	3690.2	4161.5	1.13x
200	0.9338	2179.1	2674.8	1.23x
400	0.9671	1238.6	1667.0	1.35x
600	0.9781	870.1	1255.4	1.44x
800	0.9830	665.8	1016.4	1.53x
1000	0.9858	531.0	833.4	1.57x
1500	0.9892	367.2	614.4	1.67x
2000	0.9899	272.5	459.2	1.69x