MB-36956: Move CB3ExecutorPool to own files

Move the CB3ExecutorPool class from executorpool.{cc,h} to it's own
cb3_executorpool.{cc,h} files.

Change-Id: Ie7287f7d941e060e5a83938ff82052a56c1a8dd4
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/133513
Reviewed-by: Jim Walker <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: daverigby

engines/ep/CMakeLists.txt

@@ -236,6 +236,7 @@ ADD_LIBRARY(ep_objs OBJECT
             src/bloomfilter.cc
             src/bucket_logger.cc
             src/callbacks.cc
+            src/cb3_executorpool.cc
             src/checkpoint.cc
             src/checkpoint_config.cc
             src/checkpoint_manager.cc