Clean up bitasset_data during maintenance

[pmconrad]

I just noticed that HKD (and others) contain lots of long-expired pricefeed entries. These clutter up the database and cause unnecessary burden on undo_db.
IMO we should remove expired feed entries from bitasset_data during maintenance.

[abitmore]

Please be careful, the cleanup may affect the logic here.

[abitmore]

This is actually a consensus change. Theoretically, since feed expiration period of an asset can change via asset_update_bitasset_operation, an expired feed can become valid later if hasn't been removed.

[oxarbitrage]

#598

[abitmore]

This is a behavior change, thus need a BSIP to clearly define the specifications, E.G. "how old is too old".

[xeroc]

@abitmore bitassets have a feed life time. If that timeframe is exceeded, the feed is useless and IMHO can be removed from the object

[oxarbitrage]

currently as commented by @abitmore feed_lifetime_sec and minimum_feeds can be changed by asset_update_bitasset_operation so we need a hardfork, expired feeds can became valid if they are still available.

we need to wrap #598 inside hardfork and do test cases IMHO.

not sure why a bsip will be needed here.

[pmconrad]

It's a change of behaviour / protocol change, therefore a BSIP is required.

[oxarbitrage]

ok, i'll create it tomorrow.

[oxarbitrage]

bitshares/bsips#61

[abitmore]

Done with #889. Closing.