Closes https://github.com/espressif/esp-idf/issues/4755
Closes https://github.com/espressif/esp-idf/issues/2777
Closes FCS-533
* nvs_flash_erase_partition() checks whether
the parition in question is initialized
already and will return an error if so
* reflect changes in the documentation
* nvs host unit tests now clean up after each
test case
nvs_flash: fixed deinit other partition's handles
* When deinitializing or erasing a partition,
nvs used to close all handles instead of only
the current partition's handles.
This is fixed now
* Added a unit test for that case
