kvclient, roachpb: cleanup the CPut API

The CPut API was funky: it took an expected value as a roachpb.Value,
but the checksum in it and the timestamp didn't matter. This was
undocumented. The checksum in particular covers both the key and the
value, so its relationship with a CPut is weird (since the key for
which we're using the value might be different from the key from whence
it came). This choice of Value had consequences for the Batch API:
Batch.CPut() was taking a *roachpb.Value, for which it cleared and then
re-initialized the checksum. This was all surprising, and meant that
some callers had to copy the underlying bytes for the value before
passing them to CPut().

This patch changes all the interfaces to just take []byte. There's no
longer a checksum or a timestamp to speak of. CPut() now also internally
copies the expected value's bytes, so the caller no longer needs to
worry about ownership.

Release note: None