fix: prevent refreshCfg from scheduling multiple refreshes (#666)
fix: prevent refreshCfg from scheduling multiple refreshes (#666)

## Change Description

Changes `refreshCfg` to update the `lastRefreshed` field of of the cached result once the decision to make a refresh has been made and before the cache lock is released. This fixes a race condition where more than one goroutine could attempt to perform a refresh. 


## Checklist

- [x] Make sure to open an issue as a 
  [bug/issue](https://github.com/GoogleCloudPlatform/cloudsql-proxy/issues/new/choose) 
  before writing your code!  That way we can discuss the change, evaluate 
  designs, and agree on the general idea.
- [x] Ensure the tests and linter pass
- [x] Appropriate documentation is updated (if necessary)

## Relevant issues:

- Fixes #427
GoogleCloudPlatform/cloud-sql-proxy,GoogleCloudPlatform/cloud-sql-proxy