table queries.
This change introduces a query_restrictions object into the virtual
table infrastructure, for now only holding a restriction on partition
ranges.
That partition range is then implemented into
memtable_filling_virtual_table.
This change adds a more specific implementation of the virtual table
called memtable_filling_virtual_table. It produces results by filling
a memtable on each read.
This change introduces the basic interface we expect each virtual
table to implement. More specific implementations will then expand
upon it if needed.
