* Add max_parallel parameter to MySQL backend.
This limits the number of concurrent connections, so that vault does not die
suddenly from "Too many connections".
This can happen when e.g. vault starts up, and tries to load all the
existing leases in parallel. At the time of writing this, the value
ExpirationRestoreWorkerCount in vault/helper/consts/const.go is set to
64, meaning that if there are enough leases in the vault's DB, it will
generate AT LEAST 64 concurrent connections to MySQL when loading the
data during start-up. On certain configurations, e.g. smaller AWS
RDS/Aurora instances, this will cause Vault to fail startup.
* Fix a typo in mysql storage readme
With `ha_enabled = true` vault crashes with the following error:
```
error parsing 'storage': storage.dynamodb: At 17:16: root.ha_enabled: unknown type for string *ast.LiteralType
```
This seems related to https://github.com/hashicorp/vault/issues/1559
