Use threaded version of websocket

[wch]

This should be merged after rstudio/websocket#62.

Before this change and the corresponding one in websocket, Chromote idles at ~6.5% CPU usage. After these changes, it idles at 0%.

Can be tested with:

remotes::install_github('rstudio/websocket@wch-threads')
remotes::install_github('rstudio/chromote@threaded-websocket', dependencies = FALSE)

[wch]

Now that I've thought about this a bit more, I think we can't remove the polling code just yet. It would require r-lib/later#99 to be resolved before we could make this change.

This PR will work when Chromote is used in synchronous mode, but when in async mode, it won't work at all, because nothing will be pumping the child event loop.

For example:

library(chromote)
b <- ChromoteSession$new()

# Synchronous mode
b$Runtime$evaluate("1+1") %>% str()
# List of 1
#  $ result:List of 3
#   ..$ type       : chr "number"
#   ..$ value      : int 2
#   ..$ description: chr "2"

In async mode, this results in no output:

b$Runtime$evaluate("1+1", wait_ = F)$
  then(function(value) {
    str(value)
  })

But if you run the private child event loop a few times, it will print the value:

for (i in 1:10) {
  later::run_now(0.2, loop = b$get_child_loop())
}
# List of 1
#  $ result:List of 3
#   ..$ type       : chr "number"
#   ..$ value      : int 2
#   ..$ description: chr "2"

[wch]

Closing in favor of #24.