ethclient accepts certain negative block number values as specifiers for the "pending",
"safe" and "finalized" block. In case of "pending", the value accepted by ethclient (-1)
did not match rpc.PendingBlockNumber (-2).

This wasn't really a problem, but other values accepted by ethclient did match the
definitions in package rpc, and it's weird to have this one special case where they don't.

To fix it, we decided to change the values of the constants rather than changing ethclient.
The constant values are not otherwise significant. This is a breaking API change, but we
believe not a dangerous one.

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Co-authored-by: Felix Lange <fjl@twurst.com>