fix: make deleteOldFiles / moveNewVersionInPlace more robust

[joshtrichards]

Don't do any write operations on files or direcories within RDI.

This will make the Updater more robust in FreeBSD and NFS environments (possibly others but those are the known ones).

Fixes #158

Avoids the problem that arose in #517 due to #515 by keeping the memory usage minimal by doing all the exclusions first, not directly converting the iterator to an array, and so the the resulting array sizes stay minimal.

Verified to work on FreeBSD+NFS web hosting provided by https://hostpoint.sh

Verified to work with 72,000+ objects in the apps/ folder (600+ directories containing 120 files each) without exceeding a memory_limit of 256M.

What this does:

• basically what we already do in recursiveDelete()
• most critical in deleteOldFiles() & moveWithExclusions()

Notes:

• probably would be wise to do in createBackup() & checkWritePermissions() too, but less critical since deletions/renames aren't taking place in those
• There are opportunities to do refactoring/streamlining/code clean-up but out of scope to keep this PR focused and easily reviewable.

Refs:

• https://bugs.php.net/bug.php?id=69291 (particularly the last comment)
• https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=57696

Cc: @caco3

[joshtrichards]

Test failure in master is unrelated. It's due to nextcloud/server#49224 and currently impacting all PRs in this repo.