Replace hash_gid with FNV-1a

[fuzzypixelz]

hash_gid is called on Zenoh subscriber callbacks and shows up when profiling as a hot function. The use of std::stringstream seems to be relatively inefficient. A significant portion of hash_gid's running time is spend allocating & deallocating the std::stringstream.

[fuzzypixelz]

We seem to have many uses of std::unordered_map where the hashing is done manually instead of relying on the Hash template parameter. I'll revisit that before marking this pull request as ready for review.