Fixed restore to work with disk>memory
Fixed restore to work with disk>memory

Added simple and naive eviction logic in restore code to ensure that it does not
hog memory indefinitely while running in disk>memory configuration.
Removed new thread creation on every restore file command. Now only one thread
is created for a RestoreManager instance to handle all restore file jobs.

Signed-off-by: Vandana Shah <e8ef6bf8167176b52453793987ce7a4a0f6d30eb@zynga.com>

zbase/ep-engine,zbase/ep-engine,zbase/ep-engine,zbase/ep-engine,zbase/ep-engine