"read" msg should not defer in high priority threads, can crash s4m

[iainctduncan]

Turns out read is not thread safe to defer to the low thread when in a high priority s4m object. Needs to be implemented so that it always runs in the right thread, and users will have to use alternate strategies if they want to avoid blocking (ie, read the source somewhere and send over network).

Crashes are produced when scheduler and gc are active and current read defers.

[iainctduncan]

fixed in thread branch

[iainctduncan]

merged to master, closing