fix: isolate refunded deposits from validator withdrawal queue

[matthias-wright]

Failed deposit requests have to be handled with care, because the deposit contract burns deposited funds.
If a deposit fails validation, a matching withdrawal request is queued to avoid loss of funds.
Currently, all of these withdrawal requests are grouped by the validator's public key. If an invalid deposit request triggered a withdrawal, and the same validator initiates another withdrawal, then those two withdrawals will be merged. This is not desirable, since the invalid deposit request might have different withdrawal credentials.

Changes:

• Stores withdrawals triggered by invalid withdrawal requests under a different key in the withdrawal queue
• Differentiates between deposit requests that are rejected because the validator already has a pending deposit/withdrawal request in the queue, and deposit requests that are rejected because the signatures are invalid.