The snapshot service must wait to join the child thread on exit

https://github.com/CodeChain-io/codechain/blob/18e1e80af4503cfd4d06e9514e9f88370995abda/sync/src/snapshot/service.rs#L72
Currently, the snapshot service spawns a thread to use the db but doesn't wait for the end of the child thread when it exit. It can corrupt the db.

Read sync/src/snapshot/service.rs around line 72, where the snapshot service starts its database-using child thread. Verify the exit path waits for that thread to finish, and confirm the completed service no longer risks database corruption.

Escrito por el modelo de indexación a partir del texto del issue.