Refactored atom hash into orthogonal find and add operations.

Refactored atom hash into orthogonal find and add operations.

Previously, njs_atom_find_or_add() and njs_atom_find_or_add_string()
duplicated the lookup logic and coupled find with insert. Splitting
them into njs_atom_find() and njs_atom_add() separates concerns:
find does a pure lookup with raw bytes, add inserts a pre-built value.