* app/vmalert: restore alerts state from datasource metrics
Vmalert will restore alerts state for rules that have `rule.For` > 0 from previously written timeseries via `remotewrite.url` flag.
* app/vmalert: mention remotewerite and remoteread configuration in README
* app/vmalert: initial remote-write support for alerts state persistence.
If `remotewrite.url` flag is set, vmalert will send alerts state via remote-write protocol to remote storage. The sending is asynchronous to avoid blocking calls in rules evaluation loop.
* app/vmalert: merge with master
* app/vmalert: write both `instant` and `for` alerts timeseries states in remote storage.
* Initial rules evaluation support.
Rules are now store alerts state in private field `alerts`. Every evaluation updates
the alerts and state. Every unique metric received from datastore represents a unique alert,
uniqueness is guaranteed by hashing ordered labelset.
* merge with master
* cleanup
* support endAt parameter as 3*evaluationInterval for active alerts
* make golint happy
