MFC r340404, r340415, r340417, r340419 and r340420:

MFC r340404, r340415, r340417, r340419 and r340420:
Synchronize epoch(9) code with -head to make merging new patches
and features easier.
The FreeBSD version has been bumped to force recompilation of
external kernel modules.

Sponsored by:	Mellanox Technologies

MFC r340404:
Uninline epoch(9) entrance and exit. There is no proof that modern
processors would benefit from avoiding a function call, but bloating
code. In fact, clang created an uninlined real function for many
object files in the network stack.

- Move epoch_private.h into subr_epoch.c. Code copied exactly, avoiding
  any changes, including style(9).
- Remove private copies of critical_enter/exit.

Reviewed by:	kib, jtl
Differential Revision:	https://reviews.freebsd.org/D17879

MFC r340415:
The dualism between epoch_tracker and epoch_thread is fragile and
unnecessary. So, expose CK types to kernel and use a single normal
structure for epoch_tracker.

Reviewed by:	jtl, gallatin

MFC r340417:
With epoch not inlined, there is no point in using _lite KPI. While here,
remove some unnecessary casts.

MFC r340419:
style(9), mostly adjusting overly long lines.

MFC r340420:
epoch(9) revert r340097 - no longer a need for multiple sections per cpu

I spoke with Samy Bahra and recent changes to CK to make ck_epoch_call and
ck_epoch_poll not modify the record have eliminated the need for this.