Reliable way to tell whether server is running

[king-of-poppk]

Currently, the startup message "Running plumber API at ..." is printed before this line is reached:

plumber/R/plumber.R

Line 279 in dcf7692

httpuv::runServer(host, port, self)

which makes it difficult to time when the server is actually started (think tests).

One way to fix this is to print that message (and possibly also "Running swagger Docs at ..."), or something else, between httpuv::startServer and httpuv::service here, by using those directly instead of relying on the httpuv::runServer abstraction.