ADBDEV-5478: Truncate segment files of AO relations before unlink (#675)

[silent-observer]

Truncate segment files of AO relations before unlink (#675)

Problem description:
Segment files of the AO tables were not truncated before unlink. As a result,
after relation drop, if there were file descriptors of segment files, not closed
by some backend process, disk space was not returned to the OS.

Root cause:
In mdunlinkfork, mdunlink_ao is called for an AO relation instead of truncating
all segment files. And mdunlink_ao doesn't perform truncation of segment files.

Fix:
Truncation of segment files for AO tables was added into mdunlink_ao_perFile,
which is called from mdunlink_ao for each segment file. For that purpose, static
function do_truncate was changed to global. Plus aomd unit test was updated.
Reason - unit test calls mdunlink_ao with filenames of not-existing files. Unit
test replaces unlink, so it can handle such filenames. But now
mdunlink_ao_perFile returns before invoking unlink, because do_truncate fails to
do truncation of these files. To fix it, do_truncate in aomd unit test was
replaced with a stub during the test.

Changes from original commit:

1. Added RESET for default_tablespace to previous test.

(cherry picked from commit 6f0a2c1)

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Note: do not squash to preserve authorship

[whitehawk]

In the commit description:

1. Added RESET for `default_tablespace`` to previous test.

there is an excessive quote.

[whitehawk]

Please add a not to the PR description that rebase is required (just not to forget)