Abstract storage away from filesystem about e4go

[diagprov]

For inspiration, here is the current libe4 storage API:

/* the e4storage type pre-defined above implements these API calls */
int e4c_init(e4storage *store);
int e4c_set_storagelocation(e4storage *store, const char *path);
int e4c_load(e4storage *store, const char *path);
int e4c_sync(e4storage *store);
int e4c_set_id(e4storage *store, const uint8_t *id);
int e4c_is_device_ctrltopic(e4storage *store, const char *topic);
int e4c_getindex(e4storage *store, const char *topic);
int e4c_gettopickey(uint8_t *key, e4storage *store, const int index);
int e4c_set_topic_key(e4storage *store, const uint8_t *topic_hash, const uint8_t *key);
int e4c_remove_topic(e4storage *store, const uint8_t *topic_hash);
int e4c_reset_topics(e4storage *store);

#ifdef E4_MODE_SYMKEY
int e4c_set_idkey(e4storage *store, const uint8_t *key);
#endif
#ifdef E4_MODE_PUBKEY 
/* pubkey storage apis */
int e4c_set_idpubkey(e4storage *store, const uint8_t *pubkey);
int e4c_set_idseckey(e4storage *store, const uint8_t *key);
int e4c_get_idseckey(e4storage* store, uint8_t *key);
int e4c_get_idpubkey(e4storage* store, uint8_t *key);
int e4c_getdeviceindex(e4storage *store, const uint8_t* id);
int e4c_getdevicekey(uint8_t* key, e4storage *store, const int index);
int e4c_set_device_key(e4storage *store, const uint8_t *id, const uint8_t *key);
int e4c_remove_device(e4storage* store, const uint8_t* id);
int e4c_reset_devices(e4storage* store);
int e4c_set_c2_pubkey(e4storage* store, const uint8_t* key);
int e4c_get_c2_pubkey(e4storage* store, uint8_t* key);
#endif

This is a bit peculiar to the C implementation and there is a separate find and retrieve path choice mostly because C and partly because that's how it was originally.

There will be some modifications to this, in particular I intend to provide functions to give constant pointers to certain bits of information to break the dependency between the struct and the implementation that currently exists and should not. I may also make storages check capabilities, because it's hard to enforce the right type of storage in C.

It might also evolve if we need to support better underlying data types and there is some requirement for that.