Add support for delete row

[krishvishal]

Core delete tasks:

• Implement free page functionality
• Clear overflow pages if any before deleting their cells using free page.
• Implement delete for leaf page
• Balance after delete properly

Auxiliary tasks to make delete work properly:

• Implement block coalescing in free_cell_range to reduce fragmentation.
• Track page fragmentation in free_cell_range.
• Update page offsets in drop_cell and update cell pointer array after dropping a cell.
• Add TCL tasks

Closes #455

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I will add support for balancing after delete once balance_nonroot is extended. In the current state of balance_nonroot balancing won't work after delete and corrupts page.

But delete itself is functional now.