[GAP]: Read from pre-existing file handler in `_read_block`

[tangledbytes]

Environment info

• NooBaa Version: PR
• Platform: Standalone (non containerised)

Actual behavior

PR introduced a change where we open the file to perform operations on it when TIER2 is enabled but later on close the file and then read again.

Expected behavior

It is better to not close the file if we already have a handle on it and read from the same handler instead.

[github-actions[bot]]

This issue had no activity for too long - it will now be labeled stale. Update it to prevent it from getting closed.

[tangledbytes]

@guymguym, should we close this issue (assuming we might not keep around a lot of Tier2 code for data buckets)?

[github-actions[bot]]

This issue had no activity for too long - it will now be labeled stale. Update it to prevent it from getting closed.