Autoclean for Objectcache, improved graphenrpc and threaded pingnode

Blockchainobject
* auto_clean function added. Everytime a new item is stored into the cache, all expired elements are removed.
* The __contains__ function sets now data ot None, when the date is expired, in order to reduce memory consumption
CLI
* threading for pingnode added. with --threading, all nodes are tested simultanously.
Steemnoderpc
* Error count and exception improved when only one node is available
Graphenerpc
* regests.session is stored as signelton
* requests_retry_session removed
* shared_session_instance and set_session_instance is used to get and set the global request.session object
* create_ws_instance added; A websocket is created everytime and the singleton is removed
* error_cnt and num_retries_call handling improved
RpcUtils
* sleep_and_check_retries improved
Examples
* benchmark_node2 uses threads (one thread for each node)
* memory_profiler2 added to check memory consumption for creating multiple account objects
Unit tests
* test for objectcache improved