Stringified numbers in metabase keys

[roman-khimov]

Expected Behavior

No data type conversions for DB keys.

Current Behavior

Expiration bucket using stringified representation of epoch for keys. This leads to:

• the need to strconv.FormatUint() to get the key and strconv.ParseUint() to retrieve it back (and it can return an error)
• inability to iterate in some order of epochs starting from X

Possible Solution

8 bytes BE for the same thing, ordered, fixed size, conversion that can't fail.

Context

#2535.

Likely we have more cases like this in the metabase, please add comments.