Writer requires a call to Writer::init to call content_length. This
changes prepare to correctly call init. A consequences is that
prepare can now throw unexpectedly for user-defined writers that
can fail their initialization.

This fixes a problem where a call to read() is ambiguous because
the argument list contains objects from both boost::asio and
beast::http.

Users invoking read may need to do so fully qualified, by writing:
    beast::http::read(...);

The example HTTP server is updated to provide the correct MIME-type.
It no longer uses the now-deprecated http::stream class, since that
implementation does not provide flow control. A new example async_write
function is provided in the asynchronous server for managing the
lifetime of a message sent asynchronously.

The logging is thread-safe, and a bug causing connections to
malfunction is fixed.

This solves a problem where clang and gcc locate the deleted
version of teardown and async_teardown instead of the overloaded
version. It requires overloads to add `teardown_tag` into the signature
so that the rules for argument dependent lookup can find the
right function. Improve documentation of teardown requirements

The documentation is updated to clearly explain the need for including
<beast/websocket/ssl.hpp> to use SSL streams with WebSocket.

The default implementations of teardown and async_teardown now use
static_assert to alert the user of improper usage, with comments
providing guidance for resolving the error.

in addition to passing ${CMAKE_THREAD_LIBS_INIT} to the linker, this
interface library will also add -pthread to the compile options when
supported

Signed-off-by: Casey Bodley <cbodley@redhat.com>

basic_headers no longer combines fields with the same name by appending
a comma and concatenating the two values together. This was breaking
certain header fields which expect each value to be distinct, such as
the "Set-Cookie" header.

Now the container behaves more like a multi set with respect to insertion
of multiple values with the same field name. Additional member functions
are provided to provide extra functionality.

This macro is used in the unit test framework to assist in
reporting the file and line number of test failures.

This fixes up support for building both 32 and 64 bit targets, especially on Windows.

The 'resource' parameter in the call to stream::handshake is
used when building the HTTP request to perform the upgrade.

Also fixes a bug where suite::log was incorrectly flushed.

New overloads of suite::expect take the file and line number
as individual parameters, cleaning up the file name output
by showing only the filename part (to not leak the full path,
which might contain sensitive information).

A new macro BEAST_EXPECTS allows an additional reason
string as well as reporting the file and line. Typical usage:

    ```
    error_code ec;
    ...
    if(! BEAST_EXPECTS(! ec, ec.message()))
        return;
    ```

These directories are removed, to make it easier for developers
to delete the entire directory contents when rebuilding CMake targets
after a configuration change.

* non-Windows dstream is a simple reference alias
* dstream constructor takes a target ostream argument
* dstream inherits the unitbuf setting of the target stream

git-subtree-dir: src/beast
git-subtree-mainline: 7c90b9e
git-subtree-split: 2f9a844