Add full support support for dynamic allocation and freeing of epoch's.

Make sure to reclaim epoch structures when they are freed to support
dynamic allocation and freeing of epoch structures.

While at it, move the 64 supported epoch control structures to the
static memory domain. This overall simplifies the management and
debugging of system epoch's.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D25960
MFC after: 1 week
Sponsored by: Mellanox Technologies