lib/cmap: cmap_find_batch().

lib/cmap: cmap_find_batch().

Batching the cmap find improves the memory behavior with large cmaps
and can make searches twice as fast:

$ tests/ovstest test-cmap benchmark 2000000 8 0.1 16
Benchmarking with n=2000000, 8 threads, 0.10% mutations, batch size 16:
cmap insert:    533 ms
cmap iterate:    57 ms
batch search:   146 ms
cmap destroy:   233 ms

cmap insert:    552 ms
cmap iterate:    56 ms
cmap search:    299 ms
cmap destroy:   229 ms

hmap insert:    222 ms
hmap iterate:   198 ms
hmap search:   2061 ms
hmap destroy:   209 ms

Batch size 1 has small performance penalty, but all other batch sizes
are faster than non-batched cmap_find().  The batch size 16 was
experimentally found better than 8 or 32, so now
classifier_lookup_miniflow_batch() performs the cmap find operations
in batches of 16.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>