This gets rid of the need to pass context to all cursor functions. In
practice context is always immutable when interacting with cursors.
A nicety of this is in the follow-up PR we will be able to implement
Iterator for all Cursors/filter cursors (hell, we may be able to get rid
of filter cursor altogether, as it is just a custom `filter` impl on
iterator trait).
Release Notes:
- N/A
64d0fec699