Add cfg/lookup request to server mode

[sim642]

This request allows programmatic traversal of the CFG for abstract debugging (as opposed to dumping a function's CFG in GraphViz format). The method allows two kinds of lookups:

1. by source code location,
2. by node ID.

In both cases the method returns:

1. node ID and location (useful for lookups by location);
2. next and previous nodes in the CFG.

Example session

$ rlwrap ./goblint --enable server.enabled --enable server.reparse ./tests/regression/04-mutex/01-simple_rc.c

{"jsonrpc":"2.0","id":0,"method":"analyze","params":{}}
{"id":0,"jsonrpc":"2.0","result":{"status":["Success"]}}

{"jsonrpc":"2.0","id":0,"method":"cfg/lookup","params":{"location":{"file":"tests/regression/04-mutex/01-simple_rc.c","line":18,"column":3,"byte":-1}}}
{"id":0,"jsonrpc":"2.0","result":{"node":"84","location":{"file":"tests/regression/04-mutex/01-simple_rc.c","line":18,"column":3,"byte":52985,"endLine":18,"endColumn":30,"endByte":53012},"next":[[[["Proc",null,"pthread_mutex_lock",["& mutex2"]]],"85"]],"prev":[[[["Proc",null,"pthread_create",["(pthread_t * __restrict  )(& id)","(pthread_attr_t const   * __restrict  )((void *)0)","& t_fun","(void * __restrict  )((void *)0)"]]],"83"]]}}

{"jsonrpc":"2.0","id":0,"method":"cfg/lookup","params":{"node":"85"}}
{"id":0,"jsonrpc":"2.0","result":{"node":"85","location":{"file":"tests/regression/04-mutex/01-simple_rc.c","line":19,"column":3,"byte":53016,"endLine":19,"endColumn":22,"endByte":53035},"next":[[[["Assign","myglobal","myglobal + 1"]],"86"]],"prev":[[[["Proc",null,"pthread_mutex_lock",["& mutex2"]]],"84"]]}}

TODO

• Filter synthetic locations.
• JSON-RPC error handling.
• Pseudo return nodes? Postponed to Handle pseudo return nodes in server mode cfg/lookup request #997.

/cc @FeldrinH

[karoliineh]

Seems good to me. I think we will need approval from @FeldrinH before merging this, though.

[FeldrinH]

Looks good to me.