[SPARK-4092] [CORE] Fix InputMetrics for coalesce'd Rdds

When calculating the input metrics there was an assumption that one task only reads from one block - this is not true for some operations including coalesce. This patch simply increments the task's input metrics if previous ones existed of the same read method.

A limitation to this patch is that if a task reads from two different blocks of different read methods, one will override the other.

Author: Kostas Sakellis <kostas@cloudera.com>

Closes #3120 from ksakellis/kostas-spark-4092 and squashes the following commits:

54e6658 [Kostas Sakellis] Drops metrics if conflicting read methods exist
f0e0cc5 [Kostas Sakellis] Add bytesReadCallback to InputMetrics
a2a36d4 [Kostas Sakellis] CR feedback
5a0c770 [Kostas Sakellis] [SPARK-4092] [CORE] Fix InputMetrics for coalesce'd Rdds