MFC: r333745, r333764, r337533, r339375, r341041

- ck: add support for executing callbacks outside of main poll loop
Pull in change from upstream deca119d14bfffd440770eb67cbdbeaf7b57eb7b
- Import CK as of commit deca119d14bfffd440770eb67cbdbeaf7b57eb7b.
This is mostly a noop, for mergeinfo purpose, because the relevant changes
were committed directly.
- Import CK as of commit 08813496570879fbcc2adcdd9ddc0a054361bfde, mostly
to avoid using lwsync on ppc32.
- Import CK as of commit 5221ae2f3722a78c7fc41e47069ad94983d3bccb.
This fixes two problems, one where epoch calls could occur before all
the readers had exited the epoch section, and one where the epoch calls
could be unnecessarily delayed.
- Import CK as of 21d3e319407d19dece16ee317c757ffc54a452bc, which makes its
sparcv9 atomics compatible with the FreeBSD kernel by using instructions
which access the appropriate address space.

MFC revisions r309268, r309260, r309264, r309266, r309267, r309270, r310846,
r314435, r314564, r316665, r316691, r316702.
Those import ConcurrencyKit in the FreeBSD kernel.
A few people have showed interest in this being MFC'd, so here we go.

Import Concurrency Kit in the kernel.
CK is a toolkit providing different lockfree algorithms/data structures.
More information can be found here : www.concurrencykit.org

Import concurrencykit as of commit 566bb28dba963a1904e0889b74fe7005a5bc5eb8