feat(state-keeper): pre-insert unsealed L1 batches (#2846)

## What ❔

`MempoolIO` and `ExternalIO` pre-insert unsealed batches into DB as soon
as they open them.

I chose to populate non-null fields with default values to minimize the
impact of this PR, but I am open for a discussion if anyone thinks we
need to make those fields nullable (specifically `l1_tx_count`,
`l2_tx_count`, `bloom`, `priority_ops_onchain_data`,
`initial_bootloader_heap_content`, and `used_contract_hashes`).

## Why ❔

* Better visibility of what's going on for node operators
* This opens a road to fix the issue with gas fluctuations on chains
with low amounts of traffic as `api_server` will have access to the most
up-to-date fee input

## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
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- [x] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zk fmt` and `zk lint`.