Merge pull request #237 from euroelessar/bulk_read_notsup

Call backend's bulk_read before calling several read handlers
Changed error_t to storage_error_t
Additional comment for iterate_thread_num
Moved elliptics_timer to local_session.h
Added more timers to built-in cache
Added more timer logs to cache
Moved from std::chrono to dnet_time in timer logs
Fixed elliptics_timer sign
Additional fix of signs in elliptics_timer
Added elliptics_unique_lock for more log info
Fixed log verbosity for cache erasing
Fixed syncing of all syncable objects in cache at overflow
Changed atomic from int to long to fix overflow