AsyncSource mechanism which uses a separate thread to read data can
sometimes try to manipulate an address that may be invalid because of
memory context reset or delete by the main thread.

Fix that by implementing a dedicated memory context for AsyncSource,
which is only manipulated in the reading thread.

Fixes #27.

Commit 5a1bc58 introduced a new context for AsyncSource's needs,
but failed to allocate the memory for buffer during initializion in
that context.  Fix that.

Also, add some comments.