btree: Generalize some iterator methods
btree: Generalize some iterator methods

The non-const iterator has constructor from key pointer and
the tree_if_singular method. There's no reasons why these
two are absent in the const_iterator.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla