There are actually two new client types:
- One which always connects to a given NetworkAddress, but will automatically manage a pool of reusable connections.
- One which looks up the remote address based on the URL it is given, and manages a pool of connections for each host.

The latter of these two is a "true HTTP client library".

These are useful in unit tests. Often, some tasks have been queued, and we want to test that a particular thing happens or doesn't happen as a result of those tasks, without actually waiting on any specific promise. We need a way to run the event queue until it is empty. For that, we add WaitScope::poll().

Or, sometimes, we want to check if a specific promise is (or is not!) resolved as a result of the tasks that have been queued so far, but we may not want to actulaly wait() on it for a couple reasons:
- We may actually want to verify that the promise is *not* resolved by tasks so far. wait()ing on it would be expected to deadlock.
- We may expect the promise to resolve, but would prefer not to deadlock if it doesn't resolve (we'd rather fail fast).

For this, we add Promise::poll().

There are lots of tests I've written which could be simplified by this, but for now I'm not refactoring any existing tests.

Note that adding these (particularly Promise::poll()) required updating the async framework a bit, in that this is the first case where PromiseNode::onReady() might be called multiple times, or might be canceled without destroying the PromiseNode entirely. Luckily this was not hard to account for. (We still have the rule that get() can only be called once, though.)

Make win32Socketpair() more secure

Extend kj::Network interface for easy SSRF protection

Clarify the RPC spec in some places

Add gzip (zlib) bindings.

E.g.:

    "foo"_kj

This literal will have type StringPtr. Although any string literal can implicitly cast to StringPtr, using the _kj suffix has the advantage that it can be constexpr.

ubsan: cast to unsigned integer before performing left shift

Implement TLS option to verify client certificates.

Use case for me is Cloudflare's "Authenticated Origin Pulls", in which Cloudflare authenticates itself to the origin server using client certificates. Thus the origin server can ensure that it only accepts traffic through Cloudflare, without resorting to IP whitelists.

This really is more of a data structure than a resource type, and it's useful to be able to share it between threads and whatnot.

buffers passed to PackedOutputStream::write() must be word-aligned

avoid passing invalid reference to Array<T> constructor

ubsan complains about the invalid reference even if it is unused.

RPC: Support abstract unix socket addresses on Linux