StreamPollFeeder: declare done and exception as volatile

[elharo]

StreamPollFeeder.done and StreamPollFeeder.exception are accessed from multiple threads without volatile, violating the Java Memory Model.

done (line 40): read in run() outside any synchronized block (line 61), written in waitUntilDone() inside synchronized (lock) (line 107). When the feeder thread is continuously reading data (input.available() > 0), it never enters the synchronized block, so no happens‑before guarantees the write to done is ever seen. The thread loops forever and waitUntilDone()'s join() call hangs.

exception (line 38): written by the run() thread (lines 79, 92) and read from other threads via getException() (line 101). Without volatile, a thread calling getException() may see a stale null. StreamPumper already declares its equivalent field as volatile — StreamPollFeeder should follow the same pattern.

Fixes #385
Fixes #386