AsyncClientFactory potentially calls ClusterManager::clusters() outside main thread

*Title*: AsyncClientFactory potentially calls ClusterManager::clusters() outside main thread

*Description*:
An example of this is [grpc access logger](https://github.com/envoyproxy/envoy/blob/40964cc30d4e691df5c5f0c32a449a7b4462115a/source/extensions/access_loggers/common/grpc_access_logger.h#L291).

Another example is a filter implementation similar to [ext_authz](https://github.com/envoyproxy/envoy/blob/40964cc30d4e691df5c5f0c32a449a7b4462115a/source/extensions/filters/http/ext_authz/config.cc#L84..L85) which does not skip static cluster checks (none exist in the open source codebase that I can find).

*Repro steps*:
n/a

*Admin and Stats Output*:
n/a

*Config*:
n/a

*Logs*:
see [comment](https://github.com/envoyproxy/envoy/issues/15447#issuecomment-797071949)

*Call Stack*:
n/a

related: #15447 #15227 #15449