heal: Avoid marking a bucket as done when remote drives are offline

[vadmeste]

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contributions to the project maintainers under the Apache 2 license.

Description

When the node healing an erasure set suddenly disconnects from the
cluster, len(disks) in the below line will always be empty,

disks, _ := er.getOnlineDisksWithHealing(false)

and that is because getOnlineDisksWithHealing(false) does not include
healing drives, hence the local drive will not be included in the list.

When this happens, a bucket is marked as done, which is simply wrong.

Requires at least N/2 non healing drives before deciding to start to
heal.

Motivation and Context

How to test this PR?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here