Eliminate global current epoch counter

[Alan-Jowett]

@nibanks had an interesting suggestion that is worth exploring. Currently modifying and reading Current Epoch is expensive.

Is there some scheme that can be employed to move this to a per-CPU value?

Requirements of the CE:

1. It has to be a non-decreasing value.
2. The value read by the free code must be the same or higher than the value read at the start of any currently executing RCU regions.

[dthaler]

PR #699 was one part of this investigation.