Link `flushRouterConfig` on the config server and shards

Currently there is the ability to flush router config on mongos nodes. The Primary nodes in repl sets also hold some cached config information in ChunkManager. Currently the way to 'flush' this information from a Primary is to step down, causing a new Primary to be elected. If the functionality existed that allowed a similar operation to FlushRouterConfig to run on Primaries, then this step down could be avoided in those cases where it currently is used.