Also changed the classes of reactive expressions and reactiveVal
from "reactive" and "reactiveVal" to c("reactiveExpr", "reactive")
and c("reactiveVal", "reactive")
* Rename invalidateReactiveValue to freezeReactiveValue
* Make onFlush and onFlushed use 'once' argument
* session$flushOutput: schedule another flush if needed
* Catch all errors before they propagate to websocket
* Restore original logic for progressKeys
