replace fromJSON and JSONFeed by bison parsers
Server / agent parsers have a PARSER_JSON entry to parse arbitrary Kea JSON syntax. Bison provides pure (i.e. reentrant) and push parsers too: the first one can replace fromJSON, the second the JSONFeed state machine.
I did a short benchmark (src/bin/dhcp4/tests/parser_unittest.cc diff attached) which gives for runtimes:
- 6.23s for fromJSON
- 37.8s for JSONFeed
- 4.58s for bison
Note the flex/bison files have the whole DHCPv4 syntax with Kea extended comments so large tables with only a small used subset.