Add multiPut support and other miscellaneous to PE

Add some useful stuff and some refinement to PE tool
1. Add multiPut support
Though we have BufferedMutator, sometimes we need to benchmark batch put in a certain number.
Set --multiPut=number to enable batchput(meanwhile, --autoflush need be set to false)

2. Add Connection Number support
Before, there is only on parameter to control the connection used by threads. oneCon=true means all threads use one connection, false means each thread has it own connection.
When thread number is high and oneCon=false, we noticed high context switch frequency in the machine which PE run on, disturbing the benchmark results(each connection has its own netty worker threads, 2*CPU IIRC).
So, added a new parameter connCount to PE. set --connCount=2 means all threads will share 2 connections.

3. Add avg RT and avg TPS/QPS statstic for all threads
Useful when we want to meansure the total throughtput of the cluster

4. Delete some redundant code
Now RandomWriteTest is inherited from SequentialWrite.