Suppress object transfer requests when object is already being received. (#1430)

* added deterministic check for objects received in fetch_timeout_handler.

* use receive time, in case something goes wrong after object is received.

* increase timeout for removal.

* indentation fix.

* make log info log debug. clean up debug log.

* undo unecessary changes.

* changed description var.

* shorten line 949.

* incorporate feedback.

* linting; make is_object_received function consts.

* change semantics of received_objects to objects being received.
added checks to both points at which objects are re-requested.
updated object receive initialization accordingly.

* eliminate erase on receive init. check call to request_transfer_from instead of request_transfer.

* updated comments.

* added todo for multiple object transfers.

* linting.

.gitignore

@@ -131,6 +131,7 @@ build
 # Gradle:
 .idea/**/gradle.xml
 .idea/**/libraries
+.idea
 
 # Website
 /site/Gemfile.lock