In order to eventually switch to a single JSON library,
most of the libjsoncpp usage is dropped in favor of rjson.
Unfortunately, one usage still remains:
test/utils/test_repl utility heavily depends on the *exact textual*
format of its output JSON files, so replacing a library results
in all tests failing because of differences in formatting.
It is possible to force rjson to print its documents in the exact
matching format, but that's left for later, since the issue is not
critical. It would be nice though if our test suite compared
JSON documents with a real JSON parser, since there are more
differences - e.g. libjsoncpp keeps children of the object
sorted, while rapidjson uses an unordered data structure.
This change should cause no change in semantics, it strives
just to replace all usage of libjsoncpp with rjson.
Some columns may represent not user-provided values, but ones computed
from other columns. Currently an example is token column used in secondary
indexes to provide proper ordering. In order to avoid hardcoding special
cases in execution stage, optional additional information for computed
columns is stored in column definition.
