core: more conservative cursor bounds
core: more conservative cursor bounds

I originally chose `MaxUint32` and `MaxUint64` as the bounds for the
largest possible tx position and block height, but:

1. https://github.com/chain/cp1/pull/124 says that the maximums are
actually _signed_ int32 and int64s
2. Postgres freaks out: `pq: value \"4294967295\" is out of range for type
integer` (that is `MaxUint32`. 

So let's used signed ints instead. 

Closes chain/chainprv#1896

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain