Change the partition key's representation in tokio tracing to a human-readable form

[piodul]

Currently, in RequestSpan, we are including a "routing key" - which is the partition key serialized in a particular way. The driver computes it in order to apply MurmurHash3 on it and compute the token, but it is technically also possible for users to decode a partition key from it. However, while this was simple to implement, this form is hard to parse manually and one needs to know the types of partition key's columns.

This information is available to the driver at the moment when the routing key is included in tracing, so for the sake users' convenience, the driver should decode it itself. In most of the cases it can be done without any allocations (especially after #462).