Simplify some C++

[mpoullet]

• Enable warnings when compiling.
• Read and send 16kB when using files. Why is the chunk_size set to 20kB? #48
  • change chunk_size and the delay accordingly
  • use std::make_shared to avoid calling new
  • use std::vector::data to access the underlying array
• u_char is not a standard type. Build failure on MacOSX #52
• Simplify smart pointers use.
  • prefer using std::make_shared when possible
  • use std::vector::data to access the underlying array
  • no need to call .get() on a smart pointer when comparing to nullptr
  • avoid using std::move when not required
  • use type aliasing to improve readability
  • avoid calling std::unique_ptr::reset when not needed
  • audio_output_alsa.cc: no need to put std::thread in a std::unique_ptr

[Fleker]

As we're investigating the other PR, I figured I'll go ahead with the review for this one.