Proposal: drop support for sync-freelist

[C-Otto]

With lnd 0.15+ and sync-freelist=true the database grows much faster than with 0.14, causing issues once lnd runs out of free disk space and/or the database gets too large (see below). I don't understand the details behind the setting, but I think setting sync-freelist=true doesn't fix any important issue. As such, I propose to drop the flag completely, i.e. ignore sync-freelist=true if set. If this isn't feasible, I'd like to see a warning in the configuration file (https://github.com/lightningnetwork/lnd/blob/master/sample-lnd.conf).

The node ln.nicehash.com recently ran into this issue, causing lots of force-closed channels:

https://amboss.space/node/037659a0ac8eb3b8d0a720114efc861d3a940382dcfa1403746b4f8f6b2e8810ba

Related to #6800 #6744 #6737.

[Roasbeef]

Historically we left it in place since it allows users to make a time vs space tradeoff: syncing the freelist means more writes but a faster start up, not syncing means less writes but a slower start up.

[BhaagBoseDK]

why does that impact

Historically we left it in place since it allows users to make a time vs space tradeoff: syncing the freelist means more writes but a faster start up, not syncing means less writes but a slower start up.

why should that impact the db growth?