libs/db: bbolt (etcd's fork of bolt) (#3610)

Description:

    add boltdb implement for db interface.

    The boltdb is safe and high performence embedded KV database. It is based on B+tree and Mmap.
    Now it is maintained by etcd-io org. https://github.com/etcd-io/bbolt
    It is much better than LSM tree (levelDB) when RANDOM and SCAN read.

Replaced #3604
Refs #1835

Commits:

* add boltdb for storage

* update boltdb in gopkg.toml

* update bbolt in Gopkg.lock

* dep update  Gopkg.lock

* add boltdb'bucket when create Boltdb

* some modifies

* address my own comments

* fix most of the Iterator tests for boltdb

* bolt does not support concurrent writes while iterating

* add WARNING word

* note that ReadOnly option is not supported

* fixes after Ismail's review

Co-Authored-By: melekes <anton.kalyaev@gmail.com>

* panic if View returns error

plus specify bucket in the top comment

libs/db/util_test.go

@@ -22,6 +22,11 @@ func TestPrefixIteratorNoMatchNil(t *testing.T) {
 // Empty iterator for db populated after iterator created.
 func TestPrefixIteratorNoMatch1(t *testing.T) {
 	for backend := range backends {
+		if backend == BoltDBBackend {
+			t.Log("bolt does not support concurrent writes while iterating")
+			continue
+		}
+
 		t.Run(fmt.Sprintf("Prefix w/ backend %s", backend), func(t *testing.T) {
 			db, dir := newTempDB(t, backend)
 			defer os.RemoveAll(dir)