Make AsyncUnixTest/InterruptedTimer slower on Linux becaues it turns out to be flaky on machines other than mine.

Replace all include guards with #pragma once.

@kloepper pointed out a while back that every compiler you've ever heard of supports this. Plus, it's more concise, it's not prone to copy-paste errors, and it looks nicer.

At the time I wanted to remain consistent and I didn't feel like spending the time to update all my existing code. But, every time I've added a new header since I've cursed the include guard, so I finally broke down and changed it.

Fix wrong poll/epoll timeout after EINTR.

Fix capnp_generate_cpp usage of --src-prefix

Various tweaks needed by Sandstorm HTTP changes

This gives Sandstorm a way to move the membrane to a different process when desired.

Add totalSize() to list readers, like struct readers.

Extend membrane framework to make revocation easy.

I realized upon trying to use this framework in Sandstorm that proper revocation was not easy to implement as-is. It's easy enough to revoke new calls, but canceling existing calls requires implementing a custom RequestHook and such. Since revocation is arguably the most important thing one might want to do with a membrane, this should be built-in.

Change capnp compiler to use KJ filesystem API.

Also fixed a similar _MSC_VER guard that I realized was catching MinGW by accident.

The disk-backed implementations were already inherently threadsafe. Making this explicit allows callers to rely on it.

This is particularly useful for SchemaParser, which is intended to be threadsafe.

Unfortunately, our cmake build scripts have historically passed -I flags specifying both the root of the source tree and the specific subdirectory in which the .capnp file is located. This is not correct, but since people do it (including our own cmake build), we need to not fail when this happens. Instead we print a warning.

Apparently, msync()'s parameters must be page-aligned, bah.

This allows detecting when two objects point to the same underlying file.

Fix Windows: SchemaParser::parseDiskFile() expects native paths, so use Windows-style absolute paths on Windows.

I got a little carried away here.

Otherwise, using kj::newDiskFilesystem() in the plugin complains.

This required some hairy backwards-compatibility hacks as the parseDiskFile() method is widely used.

In addition to cleaner code, this has the effect of fixing a number of issues:
- Fixes #494: Windows paths are now parsed and handled a lot better. On Windows, backslashes are handled appropriately, as are absolute paths with drive letters.
- Fixes #288: Specifing a source path starting with ".." without an appropriate --src-prefix will no longer cause output files to be generated outside of the specified output directory. Instead, the output name will be based on the absolute path, and a warning will be written since this is probably not what anyone wants.
- It's no longer necessary for --src-prefix to exactly match character-for-character. Instead, the prefix and source paths are canonicalized before matching. It even works if one is relative and the other absolute.
- Relative paths are now consistently always evaluated in path space, so ".." always cancels the previous path component and the OS/filesystem never even sees that it was there. This used to be the case in some places but not others. This behavior is important for symlink-heavy source trees: the path "foo/some-symlink/.." is now always designates "foo", rather than designating the parent directory of the symlink's target.

This change does not update the code generator plugins nor the SchemaParser API. Those will come in separate commits.

KJ_SWITCH_ONEOF should parenthesize its argument