[java] loss of precision with `number` fields

[postamar]

We should set useBigDecimals = true in the jsonSchema2Pojo gradle config.

Presently, the protocol defines fields which are oddly of type number:

• emitted_at in TRACE and CONTROL messages
• record_count

The emitted_at field is always a millisecond epoch timestamp. In Java, the jackson library will always serialize double and float values using scientific notation, which can cause loss of precision in the least significant digits, yet these are precisely those which are interesting to the user here.

The record_count was deliberately picked to be a double though the reason isn't immediately obvious, a long should be quite sufficient.

In any case, the values are always integers and not terribly huge ones either, they should be perfectly safe to represent using BigDecimal.

This change is not backward-compatible, but perhaps that's not that big of a deal? I don't know.