The Buffer API implements an append-only buffer, where you can write to
the end or read from the beginning. Data that is read is discarded from
the buffer, and you can query the buffer to get the current amount of
data inside.
Internally the buffer API is implemented as a chain of separate buffers.
This is done to avoid having to resize the existing buffer, which is
expensive. We keep track of where the reading currently is, and discard
the internal buffers after all the data in them is read.
Completions are basically a oneshot flag, which provide a `wait()` call
that blocks until 'finish()' has been called on the completion.
The nice aspect of completions is that because it is a oneshot the order
does not matter - if 'finish()' is called before 'wait()' then 'wait()'
returns immediately. It makes the logic for waiting until a thread is
done finishing up some work easy to implement.
This adds generic APIs to libqb for handling thread's, mutex's, and
condition variables. On Linux and OSX these are implemented via the ones
provided by pthreads. On Windows they're implemented via the ones
provided by the Win32 API.
For compiling, the code itself is not conditional, but the Makefile
includes logic to decide which implementation to pick.
Note that it would have been nice to simply use std::thread and friends
from C++11, however using them on MinGW appears to be a bit messy. Since
using the Windows ones directly isn't that hard this was an easy compromise.
This sets up a few different flags we'll need for the conditional
compiling, and also sets the C++ standard to gnu++11, which effectively
just matches what we were implicitly using before.
