verify preconditions during CompleteMultipart

[harshavardhana]

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Description

verify preconditions during CompleteMultipart

Motivation and Context

Bonus: hold the write lock properly to apply
optimistic concurrency during NewMultipartUpload()

How to test this PR?

Verify this by performing concurrent PUTs for both multipart and single
PUT both calls with if-none-match: * set - whoever wins first must
return an error "PrecondFailed"

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