Make unix domain socket configurable

[zhixingheyi-tian]

Now unix domain socket path string is a fixed value in code.

class ExternalCache(fiberType: FiberType) extends OapCache with Logging { private val conf = SparkEnv.get.conf private val externalStoreCacheSocket: String = "/tmp/plasmaStore" private var cacheInit: Boolean = false private var externalDBClient: ExternalDBClient = null ... }

How about making it configurable?

@winningsix @jikunshang @offthewall123

Thanks