Parse integers to ints, not float64

[trzysiek]

Standard JSON unmarshaller parses any number to float64, which causes precision errors when dealing with big integers, but still fitting into 64 bits.

One example is when we receive a JSON insert with 2^54 in it, we'll pass it to Clickhouse with some other value, which seems pretty unacceptable on production. (We have commented out test for that, look https://github.com/QuesmaOrg/poc-elk-mitmproxy/pull/269 )

It would probably also be good to check/grep other places in code, as we're using the standard unmarshaller in more than one place.