feature:support hot reloading through a multi-version InvertedDB.

[KANIOYH]

feature

issue：#51
Support hot reloading by multi-version InvertedDB.
Specifically, each request to InvertedDB logs a version number, and each time a new index is loaded, a new version is created.
The data from outdated versions is then released asynchronously.

// Manager -> versionSet:
// 	                   current ↓
// | version-1,ref:1 | version-2,ref:0 |

type Version struct {
	versionId int64 // version id, unique
	dbTable   map[string]*InvertedDB // InvertedDB  held by this version is mapped by file
	oldFiles  []string               // Files that are obsolete for the next version
	dbs       []*InvertedDB
	ref       atomic.Int64
}

Expose：
func (m *InvertedDBManager) Ref() ([]*InvertedDB, int64)
func (m *InvertedDBManager) Unref(versionId int64)
func (m *InvertedDBManager) UpdateFromRedis(ctx context.Context)

To reduce interference on the critical write path for online requests, we build versions not-in-place, minimizing the time spent on write operations.

usage

dbs, versionId := storage.GlobalInvertedDB.Ref()
defer storage.GlobalInvertedDB.Unref(versionId)
for _, inverted := range dbs {
// you code here...
}

test

mock redis chan in repository/redis/inverted_index_test.go