Factor out database handling code from ubridge resource

[prajnoha]

The ubridge resource uses kv-store resource internally for its database and it adds some more abstraction on top of the pure "kv-store":

struct kv_value {                                 
        uint64_t seqnum;
        sid_ucmd_kv_flags_t flags;
        char data[]; /* contains both internal and external data */
} __attribute__((packed));  

...and various related functions to manipulate with struct kv_value.

It would be nice if we could separate all these kv-store extras into a separate file with its own internal interface for ubridge OR alternatively even a completely new resource built on top of kv-store.