Support synchronous execution in "abaco run"

[mwvaughn]

As of Abaco 1.3.0, Abaco has a new synchronous execution mode, where a POST to an actor's mailbox will block until a response is received or timeout occurs. The result of the execution is returned as the body of the response. If bytes have been sent to the actor's /result endpoint, those bytes are returned. If not, the execution logs are returned.

Support this in abaco run as follows:

1. Add an -x option that specifies synchronous execution
2. Revise the message sending and response code to expect raw results in sync mode, no matter what the value of -v (verbose) is.

[mwvaughn]

Completed and tested pretty extensively.

[mwvaughn]

Reopening for testing