Do not reload config after every pull

[f1shl3gs]

For now, after pulling config from http provider, vertex will reload the config even if nothing is changed. It's unnecessary and waste.
We should introduce a mechanism like event's Finalizer , after the config loaded the finalizer should be called, and the behavior should be defined by each Provider. For example, http provider should have a global static variable to store a Signature of config, it could be md5sum, sha2sum or some revision like etcd's, and it works like:

1. Pull from remote server and get the signature of this config
2. Check if the new received signature is equal to the global one, if yes go to 4, or go to 3
3. Reload config, if success update the global signature, then go to 4
4. Wait next pull then go to 1