Split database to index and key-value store

[logrusorgru]

The data package represents database and keeps all inside.
We need to split it for two separate packages:

1. key-value store with API

• get []byte by hash
• set hash-[]byte pair
• increment refs count of an object by hash
• decrement the refs count

While the refs count is zero, object should be deleted. This part will be moved to server later

2. machine local index DB that keeps meta information for every object

• the size of value

• when key was created
• the number of accesses
• last access time
• ref count

[logrusorgru]

A way to handle "not found" error required.

The DB has been splited to two separate packages one of which is key-value store and can be replaced with any other package that implements following interface:

type CXDS interface {
	Get(key cipher.SHA256) (val []byte, rc uint32, err error)
	Set(key cipher.SHA256, val []byte) (rc uint32, err error)
	Add(val []byte) (key cipher.SHA256, rc uint32, err error)
	Inc(key cipher.SHA256) (rc uint32, err error)
	Dec(key cipher.SHA256) (rc uint32, err error)

	MultiGet(keys []cipher.SHA256) (vals [][]byte, err error)
	MultiAdd(vals [][]byte) (err error)

	MultiInc(keys []cipher.SHA256) (err error)
	MultiDec(keys []cipher.SHA256) (err error)
	Close() (err error)
}

TODO:

• solve the ErrNotFound problem

[logrusorgru]

Fixed by the #116