Remove dependency on sled

[romac]

Closes: #1351

Description

Remove the dependency on sled.

Caveat

For reasons which are unclear, this currently still pulls in sled even though the corresponding feature of tendermint-light-client is disabled.

I am trying to fix this by making the lightstore-sled feature disabled by default in tendermint-rs: informalsystems/tendermint-rs#977

Once the tendermint-rs is merged, the dependency on sled will disappear.

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For contributor use:

• Added a changelog entry, using unclog.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[adizere]

Neat!