Big integer numbers are stored incorrectly in json

[PiotrKorkus]

When storing one of the following numbers:

• i64::MIN
• i64::MAX
• u64::MAX
  with Rust KVS into json file, the values written to json are rounded.

e.g.
for i64::MAX we are trying to store 9223372036854775807, however 9223372036854776000 is saved in json file.

Example tests can be found:
eclipse-score/reference_integration#72