Add miner config to always keep ensealed deal copies

[magik6k]

Allows miners to set

[Sealing]
AlwaysKeepUnsealedCopy = true

in lotus-miner configs, to keep copies of deals even if those deals don't have the FastRetrieval flag set. By doing this miners can avoid having to unseal sectors while still being able to provide retrieval

[f8-ptrk]

#5435

a "never keep" flag, the opposite of AlwaysKeepUnsealedCopy = true would be nice, not sure if it needs a second flag or another one for "let the client choose" then

[f8-ptrk]

in addition: it might be worth thinking about an extra folder for these "sealed"-unsealed sectors so miners can link storage to the folder that doesn't need to be local and does not interfere with the "normal" unsealed folder

we have #5291 open - i think it could extend that to maybe even have a totally separated folder structure for deal related data?

[magik6k]

in addition: it might be worth thinking about an extra folder for these "sealed"-unsealed sectors so miners can link storage to the folder that doesn't need to be local and does not interfere with the "normal" unsealed folder

Those unsealed files get moved to long term storage (--store) in the Finalize step

[magik6k]

(will add the other option in a separate PR)