Provide a configuration to adjust the weight of EC recovery tasks to adjust the speed of recovery

The relative speed of EC recovery comparing to 3x replica recovery is a function of (EC codec, number of sources, NIC speed, and CPU speed, and etc).

Currently the EC recovery has a fixed xmitsInProgress of max(# of sources, # of targets) comparing to 1 for 3x replica recovery, and NN uses xmitsInProgress to decide how much recovery tasks to schedule to the DataNode this we can add a coefficient for user to tune the weight of EC recovery tasks.