* Add a new `silence import` command to amtool
This command read silences data from a query JSON output and import to
alertmanager. It allows `amtool` to be used as a backup/restore tool for
silences, i.e. #1000
Backup / export:
```
amtool silence -o json > silences.json
```
Restore / import:
```
amtool silence import silences.json
```
* Add a WaitGroup barrier
Move error channel reading to a goroutine to prevent deadlock and thus
add a WaitGroup to synchronize.