BlockingClientCall.write should block after writing the message

When a program calls `BlockingClientCall.write`, it's already done the work necessary generate a message. Little is lost then by immediately serializing the message and preparing it for transmission once the flow control window opens up. `BlockingClientCall.write`, however, waits for the call to be ready before taking any steps to send the message. I propose to move the blocking to after calling `ClientCall.sendMessage`.