database/sql/driver: rename ResetSessioner to SessionResetter
Originally we tried the strict -er suffix as the rule in this case
but eventually we decided it was too awkward: io.WriteByter
became io.ByteWriter. By analogy, here the interface should be
named SessionResetter instead of the awkward ResetSessioner.
This change should not affect any drivers that have already
implemented the interface, because the method name is not changing.
(This was added during the Go 1.10 cycle and has not been
released yet, so we can change it.)