Reduce bytes / hex conversions of content keys

[ScottyPoi]

Issue:

Excessive need to use hexToBytes and bytesToHex conversion methods on contentKey, contentId, blockHash, and more.

Cause:

Inconsistent design of functions. Some functions expected keys to be strings, others expected Uint8Arrays.

Solution:

Internally, contentKeys should always be Uint8Array, with the only conversion to and from strings happening in the DB manager.

contentId should always be an unprefixed hex string, matching the type of enr.nodeId, and the expected input of the imported distance function distance(nodeId, contentId)

[acolytec3]

LGTM - I salute your attention to details