Integrate python json parser as a service for testing

[moticless]

• One of the outputs of librdb is JSON. Until now there was a naive testing code that compared two json files, attempting to ignore JSON elements ordering. This PR replaces it with a python service, run in the background, read JSON filename from its STDIN, generates its signature by ignoring JSON elements order, and prints SHA256 signature to its STDOUT. test_common.c will fork this service. The reason it is running as a service is to avoid the overhead of starting a new process for each evaluation and the intensive use of it.

• In addition the optional json fields will be now in the form:

__aux__ : { …. }
__slotinfo__ : { …. }
__dbsize__ : { …. }

• In JSON, the value “-0.0” is a valid double, but we want to output it as 0.

• Modified librdb tests to run with -O0 optimization instead of -O3

[tezc]

LGTM. Just added minor comments.