Add option to EXEC_PATH_DISPLAY to display also execution nodes tree

[aafemt]

An improvement for EXEC_PATH_DISPLAY. It is more convenient to see the execution tree in ISQL instead of debug log only.

XML formatting of the nodes wasn't changed from original implementation.

[asfernandes]

The XML node-tree is a development feature with missing things.
I'm against put it in production in the current state.
Also it prints internal details (numeric constants) that we do not want to stabilize.

[aafemt]

So does BLR printer. This feature is for debugging, not production.

[asfernandes]

BLR is stablished thing, it's API.
Emitted BLR should always be parseable and there were already BLR-to-text filters of stored routines.

[aafemt]

Note: this pull request doesn't change the node printer, just add a new way to receive the result. If the node printer is incomplete and messy - it is a separate issue and should be fixed by separate PR(s).