Prior art: T41470: Can't merge blocked users: Duplicate entry for key 'ipb_address'

The potential workflow if there are block_target for the source account:

Step 1: update bt_user and bt_user_text in block_target

• If there are previously no block_target row with the target account, we are done.
• If there are block_target row and no block row for the target account, drop the block_target row and we are done.
• If there are block_target row and block row for the target account, drop the block_target row and go to Step 2 to fix the block row.

Step 2: update block row

• First we change the bl_target to the new block row
• How to update block row depends on how many blocks are allowed (see T351508: Decide the limit of multiblocks)
  • If we support arbitrary number of blocked: we may (or must, depends on T351511) remove redundant blocks.
  • If we support a limited number of block: first we may (or must, depends on T351511) remove redundant blocks, then we need to sort the blocks somehow and drop some blocks.
  • If we only support one block the current way for block seletion may be used.