Turn on epoll changelists with libevent 2.0.9-rc and later

Libevent 2.0 has a "changelist" feature that avoids making redundant
syscalls if we wind up doing a lot of event_add/event_del operations
on the same fd in a row.  Unfortunately, due to a weird design
choice in Linux, it doesn't work right with epoll when multiple fds
refer to the same socket (e.g., one is a dup() of the other).  We
don't dup() anything we give to Libevent, though, so it is safe for
us to explicitly turn this feature on.

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