Made delete_at_term_t permit start == end == size.  See #2696.
Merge branch 'next' into sam_2696

Conflicts:
	src/rdb_protocol/datum.hpp
Added polyglot tests for #2696 delete_at term fixes.
Merge branch 'sam_2696' into next
merged #2822 import_python_driver fix
Merge branch 'next' into sam_2821
merge of #1712 fix spelling on import_pyton_driver
Merge branch 'next' into sam_2821