Server should send progress and busy messages to client

[wch]

Currently, pyshiny does not send busy and idle messages to the client.

Here's a log of the communication in R-shiny:

SEND {"config":{"workerId":"","sessionId":"da408435a4b00fe1731b0926eda7bc49","user":null}}
RECV {"method":"init","data":{"n:shiny.number":1,".clientdata_output_txt_hidden":false,".clientdata_pixelratio":2,".clientdata_url_protocol":"http:",".clientdata_url_hostname":"127.0.0.1",".clientdata_url_port":"7453",".clientdata_url_pathname":"/",".clientdata_url_search":"",".clientdata_url_hash_initial":"",".clientdata_url_hash":"",".clientdata_singletons":""}}
SEND {"busy":"busy"}
SEND {"recalculating":{"name":"txt","status":"recalculating"}}
SEND {"recalculating":{"name":"txt","status":"recalculated"}}
SEND {"busy":"idle"}
SEND {"errors":{},"values":{"txt":"input$n is 1"},"inputMessages":[]}
RECV {"method":"update","data":{"n:shiny.number":5}}
SEND {"progress":{"type":"binding","message":{"id":"txt"}}}
SEND {"busy":"busy"}
SEND {"recalculating":{"name":"txt","status":"recalculating"}}
SEND {"recalculating":{"name":"txt","status":"recalculated"}}
SEND {"busy":"idle"}
SEND {"errors":{},"values":{"txt":"input$n is 5"},"inputMessages":[]}

And a similar one in py-shiny:

SEND: {"config": {"workerId": "", "sessionId": "1", "user": null}}
RECV: {"method":"init","data":{"n":20,".clientdata_output_txt_hidden":false,
".clientdata_pixelratio":2,".clientdata_url_protocol":"https:",".clientdata_
url_hostname":"rstudio.github.io",".clientdata_url_port":"",".clientdata_url
_pathname":"/prism-experiments/app-b5nq8bxz2uydkuf2y7wk/",".clientdata_url_s
earch":"",".clientdata_url_hash_initial":"",".clientdata_url_hash":"",".clie
ntdata_singletons":""}}
SEND: {"recalculating": {"name": "txt", "status": "recalculating"}}
SEND: {"recalculating": {"name": "txt", "status": "recalculated"}}
SEND: {"errors": {}, "values": {"txt": "n*2 is 40"}, "inputMessages": []}
RECV: {"method":"update","data":{"n":49}}
SEND: {"recalculating": {"name": "txt", "status": "recalculating"}}
SEND: {"recalculating": {"name": "txt", "status": "recalculated"}}
SEND: {"errors": {}, "values": {"txt": "n*2 is 98"}, "inputMessages": []}

[cpsievert]

For future reference, the logic for the "binding progress" message is here https://github.com/rstudio/shiny/blob/78d77ce3733d947b292614a626e04ca5167aa445/R/shiny.R#L1306-L1323