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.