distsqlrun: reduce memory account overhead in merge joiner

[EasyVector]

Profiling the merge joiner reveals that we spend almost 20% of our time manipulating memory accounts: We can almost certainly do better here, and potentially see some major performance gains by doing so.

[issue-mula-labeler[bot]]

MULA Label Bot automatically applied the label performance to this issue, with a confidence of 1.00. Please mark this comment with :thumbsup: or :thumbsdown: to send your feedback!

[issue-mula-labeler[bot]]

MULA Label Bot automatically applied the label enhancement to this issue, with a confidence of 1.00. Please mark this comment with :thumbsup: or :thumbsdown: to send your feedback!

[issue-mula-labeler[bot]]

MULA Label Bot automatically applied the label database to this issue, with a confidence of 1.00. Please mark this comment with :thumbsup: or :thumbsdown: to send your feedback!

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label client to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label server to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label document to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label security to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label design to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label question to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label ui to this issue, with a confidence of 0.00..

[issue-mula-labeler[bot]]

MULA Label Bot is not confident enough to apply the label bug to this issue, with a confidence of 0.00..