os: IndexManager:: optimize map<coll_t, CollectionIndex* > col_indices to unordered_map

col_indices is just used to cache CollectionIndex, and use col_indices.find() to get its corresponding index. Without any using of up_bound() and low_bound(), we can use unordered_map to make it fast. Based on perf, when I chanage map to unordered_map, the cpu cost for get_index(coll_t c, const string& baseDir, Index *index) is much lower.

Signed-off-by: Ning Yao <zay11022@gmail.com>