HTTP/2 Unit Tests EventLoopGroup cleanup

Motivation:
The HTTP/2 unit tests are suffering from OOME on the master branch.
These unit tests allocating a large number of threads (~706 peak live) which may
be related to this memory pressure.

Modifications:
Each EventLoopGroup shutdown operation will have a `sync()` call.

Result:
Lower peek live thread count and less associated memory pressure.
Fix Native EPOLL Build Failure

Motiviation:
If sendmmsg is already defined then the native epoll module failed to build because of conflicting definitions.
The mmsghdr type was also redefined on systems that already supported this structure.

Modifications:
Provide a way so that systems which already define sendmmsg and mmsghdr can build
Provide a way so that systems which don't define sendmmsg and mmsghdr can build

Result:
The native EPOLL module can build in more environments