Reset the digester of the follower at the beginning of each file transfer during a InstallSnapshot to avoid snapshot transfer failure

Digester of the Follower only calls `digester.get().digest()` to get an MD5 and reset the engine when a file is completed. However, if the client does not complete the file transmission when the streaming time expires, digester is not reset during subsequent snapshot transfers. Therefore, a different md5 value is calculated, causing the installSnapshot to fail forever.