release-23.2: restore: limit restore spans to 200 files

[blathers-crl]

Backport 1/1 commits from #119840 on behalf of @dt.

/cc @cockroachdb/release

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Previously we would limit restore spans to 384MB, but if a backup contained many small files, a large number of these files could be grouped into such a span before it hit the 384MB target. Unfortunately this caused the restore process to open all of those files to process the restore span, leading to very large numbers of concurrent connections when restoring a backup consisting of tiny files.

This patch limits the restore spans to 384MB or 200 files, whichever is hit first. Restores of backups with large numbers of tiny files may thus be slightly slower but will better limit their concurrent outbound connections.

Fixes #119785.

NB: This breaks the test cases for the experimental/disabled support for memory-monitored restores. Given we have no plans to actually enable these at this time, but do need to fix this bug in the normal restore path and backport such a fix, to reduce churn in said backports the tests of the disabled mode are simply skipped in this diff rather modified, in anticipation of removing them completely from the development branch, tracked in #119836.

Release note (bug fix): fix a bug where RESTORE on certain backups would open a very large number of concurrent connections to the backup storage provider.

Epic: none.

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Release justification: bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[jbowens]

lgtm