Extract signal trapping to SignalTrapper

1. There were some ugly `rescue` clauses for when the environment doesn't support a signal. I simplified this code by extracting the logic into a new helper, `SignalTrapper`.

2. The code for trapping the `CONT` signal was refactored a long time ago, but it looks like a line of legacy code was accidentally left in: `Worker` was trapping `CONT` to call `unpause_processing` in a block, but that method hasn't existed for some time. I removed this line, so now the only class that traps that signal is `IOAwaiter`.