components/nvs: handle more cases where sudden power off may happen

This commit fixes several issues with state handling in nvs::Page. It also adds extra consistency checks in nvs::PageManger initialization.
These changes were verified with a new long-running test ("test recovery from sudden poweroff"). This test works by repeatedly performing same pseudorandom sequence of calls to nvs_ APIs. Each time it repeats the sequence, it introduces a failure into one of flash operations (write or erase). So if one iteration of this test needs, say, 25000 flash operations, then this test will run 25000 iterations, each time introducing the failure point at different location.

components/nvs_flash/include/nvs.h

@@ -36,6 +36,7 @@ typedef uint32_t nvs_handle;
 #define ESP_ERR_NVS_NOT_ENOUGH_SPACE    (ESP_ERR_NVS_BASE + 0x05)
 #define ESP_ERR_NVS_INVALID_NAME        (ESP_ERR_NVS_BASE + 0x06)
 #define ESP_ERR_NVS_INVALID_HANDLE      (ESP_ERR_NVS_BASE + 0x07)
+#define ESP_ERR_NVS_REMOVE_FAILED       (ESP_ERR_NVS_BASE + 0x08)
 #define ESP_ERR_NVS_KEY_TOO_LONG        (ESP_ERR_NVS_BASE + 0x09)
 #define ESP_ERR_NVS_PAGE_FULL           (ESP_ERR_NVS_BASE + 0x0a)
 #define ESP_ERR_NVS_INVALID_STATE       (ESP_ERR_NVS_BASE + 0x0b)
@@ -92,6 +93,10 @@ esp_err_t nvs_open(const char* name, nvs_open_mode open_mode, nvs_handle *out_ha
  *             - ESP_ERR_NVS_INVALID_NAME if key name doesn't satisfy constraints
  *             - ESP_ERR_NVS_NOT_ENOUGH_SPACE if there is not enough space in the
  *               underlying storage to save the value
+ *             - ESP_ERR_NVS_REMOVE_FAILED if the value wasn't updated because flash
+ *               write operation has failed. The value was written however, and
+ *               update will be finished after re-initialization of nvs, provided that
+ *               flash operation doesn't fail again.
  */
 esp_err_t nvs_set_i8  (nvs_handle handle, const char* key, int8_t value);
 esp_err_t nvs_set_u8  (nvs_handle handle, const char* key, uint8_t value);