This CL introduces a Recycler to reuse unreferenced vk::Fence, reducing
CPU time spent on vk::Fence init&destroy. Save around 15% of CPU time
for most glmark2 tests.

Bug: angleproject:3556
Change-Id: Ice5054305321c466c5be3bc368d04091f074729c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1679239
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>