[BUG] TiFlash is assumed to be enabled by default

[ankita25]

Describe the bug
TiFlash is assumed to be enabled by default and hence a threadpool is created at application start up to check tiflash replica status which causes null pointer exception
What did you do
Deployed tidb without tiflash . Ran tidb MR connector on it
What do you expect

No error regarding tiflash since its not deployed
What happens instead

21/11/29 20:56:49 INFO common.PDClient: get tiflash table replica sync progress failed, continue checking.
java.lang.NullPointerException
at org.tikv.common.PDClient.updateTiFlashReplicaStatus(PDClient.java:572)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
21/11/29 20:56:49 INFO common.PDClient: get tiflash table replica sync progress failed, continue checking.
java.lang.NullPointerException
at org.tikv.common.PDClient.updateTiFlashReplicaStatus(PDClient.java:572)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
21/11/29 20:56:49 INFO common.PDClient: get tiflash table replica sync progress failed, continue checking.
java.lang.NullPointerException
at org.tikv.common.PDClient.updateTiFlashReplicaStatus(PDClient.java:572)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)

Java Client and TiDB/TiKV version info
testing #342