MB-36956: Allow multiple ExecutorPool backends

To facilitate using a Folly executor-based ExecutorPool instead of our
Global Executor added in Couchbase 3.0, rename the existing
ExecutorPool class to 'CB3ExecutorPool', ExecutorThread to
CB3ExecutorThread, and make ExecutorPool a pure virtual interface.

Currently this still creates exactly the same CB3-style ExecutorPool,
it just adds an indirection to permit other implementations in future.

Change-Id: Iaded9719a832e529147c2e5a61f72095e6f8360b
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/130418
Tested-by: Build Bot <[email protected]>
Reviewed-by: Jim Walker <[email protected]>

Author: daverigby

engines/ep/benchmarks/executor_bench.cc

@@ -28,6 +28,7 @@
 #include <folly/executors/CPUThreadPoolExecutor.h>
 #include <folly/executors/IOThreadPoolExecutor.h>
 #include <folly/futures/Future.h>
+#include <relaxed_atomic.h>
 #include <random>
 
 /**
@@ -40,7 +41,6 @@ class ExecutorBench : public benchmark::Fixture {
         // ExecutorPool cannot handle having a count of zero :(
         pool = std::make_unique<TestExecutorPool>(
                 nonIOCount,
-                NUM_TASK_GROUPS,
                 ThreadPoolConfig::ThreadCount(1),
                 ThreadPoolConfig::ThreadCount(1),
                 1,