Simplify threading model in REPL proxy

[0x2c7]

Recently, the dance between REPL threads is kinda complicated than it needs. There used to be 4 threads involve: 1 thread for Pry REPL loop, 1 thread for flushing output from REPL, 1 thread for listening to keyboard, and the stopping thread to sleep, and control states. It's a flawed solution, as the input in REPL loop is evaluated in another thread other from the stopping thread. It leads to many unexpected behaviors, like errors when accessing thread-local variables. It's time to standardize, and evaluation to the stopping thread instead.

Fix #73