Switch internal agent selector to improve throughput

[sagebind]

libcurl uses poll(2) at best case for its selector implementation, and at worst case uses select(2), which can perform rather badly with many concurrent requests. Isahc already performs really well (at least under unix-like systems), but to get maximum possible performance, we should switch away from using curl_multi_select() and switch to using a library that efficiently uses the best selector available on the target system. This should especially aid performance on Windows, where IOCP has far better throughput than select(2).

Initially we investigated using Mio which has many nice benefits, but unfortunately will not work for us since Mio requires ownership of the socket handles and requires all read and write calls to go through Mio. But libcurl is going to read and write to the socket handles directly and we can't change that, so Mio is out.

Sometime after this issue was initially created the polling crate was introduced, which looks very promising and should work for us as an alternative. It has less features than Mio, but frankly we don't need those extra features, and more importantly polling allows us to keep ownership of the socket handles. On Windows it uses wepoll which is a widely accepted readiness-driven wrapper around IOCP that performs well.

Implementation is slow-going, but is on this PR here: #243

[sagebind]

It appears that this cannot be done on Windows without modifying libcurl to make less assumptions with its socket management. Closing this for now, as it isn't worth the high effort for an unknown amount of efficiency improvement.

In particular, IOCP on Windows requires you to use specific read and write functions instead of the standard ones, which libcurl doesn't allow you to configure.

[sagebind]

Re-opened, since we finally have a potential way forward using the excellent polling crate.