nvs: do eager cleanup of HashListBlocks

nvs: do eager cleanup of HashListBlocks

Previously when HashList was removing items, HashListBlocks were
removed lazily. This resulted in empty HashListBlocks dangling around
in full pages, even when all items have been erased. These blocks
would only be deleted when NVS was re-initialized
(nvs_flash_deinit/nvs_flash_init).

This change does eager cleanup instead, based on the code from
@negativekelvin offered in
https://github.com/espressif/esp-idf/issues/1642#issuecomment-367227994.

Closes https://github.com/espressif/esp-idf/issues/1642.