Fix several instances of PulseAudio API misuse

[kinetiknz]

• Make sure we hold the mainloop lock when accessing PA objects that require the lock held
• Handle get_sink_info_by_name failure to avoid potential deadlock on startup with misconfigured PA servers
• Fix Device/ffi::cubeb_device mismatch between current_device/device_destroy, also handles a string leak on the error path
• Introduce MainloopLockGuard

[padenot]

A question on the locks, the rest looks fine.

[kinetiknz]

This also fixes the case where we unlocked the mainloop lock too early if we entered the log_enabled() path.