Explain analyze with explain_memory_verbosity set to 'detail' or 'summary' will show total memory used accross slices
Explain analyze with explain_memory_verbosity set to 'detail' or 'summary' will show total memory used accross slices

We will be adding an extra line detailing total memory used across slices to explain analyze. This will help us in
profiling memory usage from TPC-DS benchmark.

```
(slice0)    Executor memory: 322K bytes.  Peak memory: 11783K bytes.  Vmem reserved: 12288K bytes.
   (slice1)    Executor memory: 8375K bytes avg x 3 workers, 8375K bytes max (seg0).
  Peak memory: 9488K bytes avg x 3 workers, 9488K bytes max (seg0).  Vmem reserved: 9216K bytes avg x 3 workers, 9216K bytes max (seg0).
 Total memory used across slices: 40247K  bytes
 Statement statistics:
   Memory used: 128000K bytes
```

Signed-off-by: Sambitesh Dash <0c774551a12bd2c108d5895626002000c1bb7d82@pivotal.io>

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