Replace Database::CollectionMap(std::map) with std::unordered_map

[holiday-404]

The map is used for giving a collection name and returning a collection handler.

abseil is a git submodule of txservice. We should make abseil a system library first.

[xiexiaoy]

absl::flat_hash_map requires ValueT to be copyable.
std::unordered_map doesn't support Heterogeneous Lookup.
None of them works.