Shift to another hasmap implementation from the deprecated c_hashmap

[rootkea]

Hello!

Currently, we are using c_hashmap https://github.com/petewarden/c_hashmap which has been deprecated and suggests https://github.com/sheredom/hashmap.h as a replacement.

From https://github.com/petewarden/c_hashmap/blob/master/README:
"This version is now deprecated, since it's unmaintained. @sheredom has a newer version at https://github.com/sheredom/hashmap.h"

I researched a bit and these are some of the popular hashmap implementations:

1. https://github.com/sheredom/hashmap.h (suggested by c_hashmap)
2. https://github.com/skarupke/flat_hash_map
3. https://github.com/greg7mdp/parallel-hashmap
4. https://github.com/khizmax/libcds

IMHO, we should move to any of these or any other active hashmap implementation from the current deprecated c_hashmap.

[rootkea]

I can take this one if https://github.com/sheredom/hashmap.h sounds as an apt alternative.

[cxong]

Sure, happy to review a PR that implements this, but please make sure we regression test all the places that use hashmaps