Make thread pool for submitting range deletion tasks a decoration on ServiceContext

This would make it possible to shutdown and restart the migrationutil::getMigrationUtilExecutor() thread pool between C++ test cases. RangeDeleterTest::tearDown() in range_deletion_util_test.cpp is currently problematic because it doesn't prevent a task from starting to run while the ServiceContext is being destroyed, nor does it prevent a task from starting to run while the (static storage duration) logv2 subsystem is being destroyed.