make BlockingKernelClient the default Client

the default Client class returned by `KernelManager.client()`
should be BlockingKernelClient, because the base KernelClient is incomplete.

Ultimately, the base KernelClient will probably become a blocking client,
and the Blocking subclass will go away.
remove redundant BlockingKernelClient import from test_message_spec
Merge pull request #3222 from minrk/blockingdefault

make BlockingKernelClient the default Client