Serving stop periodically reloading feature specification after a while

## Expected Behavior 
Serving will periodically reload feature spec by calling core

## Current Behavior
After a while serving is not reloading its feature spec cache

## Steps to reproduce
Run feast normally and observe number of get features from serving to core is zero after awhile

### Specifications

- Version: 0.1.0 , 0.1.1
- Platform: any
- Subsystem: Serving