Refactor and simplify isc_symtab

Refactor and simplify isc_symtab

This commit does several changes to isc_symtab:

1. Rewrite the isc_symtab to internally use isc_hashmap instead of
   hand-stiched hashtable.

2. Create a new isc_symtab_define_and_return() api, which returns
   the already defined symvalue on ISC_R_EXISTS; this allows users
   of the API to skip the isc_symtab_lookup()+isc_symtab_define()
   calls and directly call isc_symtab_define_and_return().

3. Merge isccc_symtab into isc_symtab - the only missing function
   was isccc_symtab_foreach() that was merged into isc_symtab API.

4. Add full set of unit tests for the isc_symtab API.