Replaced Timer::create/cancel with Clock::timer/cancel.
Updates to Mesos for Timer::create/cancel -> Clock::timer/cancel.
Abstract clock internals from ProcessManager::settle.
Introduced a callback for timer expiration.
Removed redundant synchronization and conditional check.
Simplified redundant Clock::order/update usage.
Introduced a libev async watcher specifically for updating timers.
Moved Clock implementation into clock.cpp.
Started moving libev specific functionality out of process.cpp.
Moved event loop specific polling into poll.cpp.
Moved process::io::* into io.cpp.
Used io::poll instead of libev for recv_data.
Used io::poll instead of libev for ignore_data.
Used io::poll instead of libev for receiving_connect.
Used io::poll instead of libev for sending_connect, send_data, send_file.
WIP: add connect to socket. There's a race condition with send's connect path.