[2.x] meta.Client for a kv.Store needs to write lock when reloading data into the client's data cache

The `meta.Client` keeps a tsdb cache of all the underlying `kv.Store` data (updates perform a full flush to disk with a Bolt implementation of a `kv.Store`).

The `Client` has a public `Load` method to refresh the cache from the `kv.Store` - as this updates the `Data` cache, it needs a write lock. The current `Load` method does not lock opening a window for corruption or a bad read from the cached data. It may be a possible bug instead of a latent bug. There are two call sites of `Load': one is under a write lock (Open) and the other other is likely problematic as it is used by the `RestoreKVStore` endpoint and cli `influx restore`.

https://github.com/influxdata/influxdb/blob/0dc48b12609d840b86cca439476e106fc2692d44/v1/services/meta/client.go#L991-L992