Executor abstraction (#384)

This change adds a layer of abstraction over poolmgr. Poolmgr is now just one of the ways to turn a function into a service; other implementations will be added. The executor abstraction is a uniform API over all these implementations.

* Executor layer added on top of pool manager

* Removed the external server for executor

* Minor changes to keep existing semantics as much possible

* Separating the executor vs. poolmgr backend functionality and associated data members

* Executor logic separated from Poolmgr backend completely, placeholder for new backend

* Changed references to poolmgr in tests

* Moved poolmgr to it's package, as a side effect moved Cache to its's package (was causing cyclical dependency) and had to make some data structures exposed outside package

* Rebased from master and changed references to tpr -> crd

* Executor layer added on top of pool manager

* Executor logic separated from Poolmgr backend completely, placeholder for new backend

* Changed podName to a generic objectReference in fscache (#391)

Changed podName to a generic objectReference in function service cache implementation.

* Moved poolmgr to it's package, as a side effect moved Cache to its's package (was causing cyclical dependency) and had to make some data structures exposed outside package

* Rebased from master and changed references to tpr -> crd

* Merged from master with latest changes

* Removed stale executor service & deployment from previous merge

* Addressed review comments, still testing some areas

test/test_utils.sh

@@ -262,8 +262,8 @@ dump_logs() {
     dump_env_pods $fns
     dump_fission_logs $ns $fns controller
     dump_fission_logs $ns $fns router
-    dump_fission_logs $ns $fns poolmgr
     dump_fission_logs $ns $fns buildermgr
+    dump_fission_logs $ns $fns executor
     dump_function_pod_logs $ns $fns
     dump_fission_crds
 }