Get rid of unnecessary calls to RouterSelectTask() for INSERT ... SELECT

In the current implementation of `INSERT ... SELECT` queries, the following algorithm is followed:

```python

for targetShardInterval in shardIntervalsOfInsertedTable:

InstantiateQualToTargetShardInterval()
call RouterSelectTask()
call ShardPruning() on all shards
```

The above algorithm does not scale for high-shard counts. We should probably implement this more efficient.

One possible approach could be (as prototyped in #1240 ):

```python

prunedShardList = PruneShardList()

for targetShardInterval in shardIntervalsOfInsertedTable:

if targetShardInterval not in prunedShardList:
continue;

InstantiateQualToTargetShardInterval()

call RouterSelectTask() with input shard list as prunedShardList (instead of the whole shards)
call ShardPruning() with input shard list as prunedShardList (instead of the whole shards)
```

This could dramatically decrease the planning time with high shard counts where the query hits only few shards.