Add versioning to the libraries in the CMake build

The Autotools and CMake builds are almost the same, except for
the libraries not being generated as versioned libraries in CMake.

Fixing this lets Linux CMake builds be on the same level as Autotools ones.

Add cmake_minimum_required to root CMakeLists.txt

There was already a minimum version specified in the actual CMakeLists
file in the C++ directory, but since the parent file does not have it
you get a warning if you build in the idiomatic cmake fashion:

    mkdir build
    cd build
    cmake .. # <---- warning here because no minimum version specified

I used 3.1 as min version because that is the same as specified in child
file.

This reverts commit b50f6598.

Merge branch 'doccomments' of https://github.com/opensourcerouting/capnproto into opensourcerouting-doccomments

Fix MSVC build error when using List<T> with generic T

Add CMakeLists.txt to top level

Tweaks and bugfixes

It turns out that, quite often, the write() will have already completed before the method returns, hence dropping the returned promise won't cause any apparent problem -- until you try to write() a sufficiently large value that it doesn't complete on the first call. Then, you get mysterious bugs.

Indeed, it turns out the HTTP library has such bugs, which are fixed in this change.

std::initializer_list is problematic because it insists that its elements be const, meaning among other things that you can't move from them. So, an std::initializer_list<kj::String> is often useless. With kj::arr you can do like:

    kj::Array<String> = kj::arr(kj::mv(string1), kj::mv(string2));

This requires C++17 due to the fold expression. This could maybe be worked around using some ugly recursion but I'm writing C++17 these days so meh.

I'm increasingly thinking that maybe kj::Array itself should support Vector semantics, but for now this change makes it easier to write code that uses Vector instead of Array.

Actually add URL library to makefiles

Add URL parsing library to libkj-http

Closes #479.

If T is a template instantiation and `List<T, kind<T>()>` is the return or
parameter type of a function (notably get, set, init, adopt, and disown
functions) defined out-of-line from its definition, MSVC fails to match
the definition with its declaration. This seems to be another consequence
of poor expression SFINAE / constexpr support.

An easy workaround is to avoid using kind<T>() and instead manually
instantiate Kind_<T>, as in lite mode. When I converted Kind_<T> to use
VoidSfinae, I had thought this solved the problem, but clearly there are
still edge cases.

This work-around had been removed in 75c7aaee.

However, it turns out that many Debian Jessie systems still use 4.9.2 (not 4.9.4, which is fixed).

I've changed the #if to more specifically target 4.9.x, which should cover all affected versions (in addition to some fixed versions).

This uncovered some bugs and revealed that there was no way to read the Content-Length of a HEAD response. Fixed.

Although contrary to conventions, it's surprisingly easy to write code that calls connect() and then drops the address before the connection completes, and such code works with the default network implementation, so let's accept it.