wallet: immediate lookahead table expansion + set_subaddresss_lookahead RPC endpoint [0.18]

[nahuhh]

release version of #9953 / #8981

[nahuhh]

I've tested wallet-cli's set subaddress-lookahead and it works as expected.

things to note:

• this does not work retroactively. If you missed any transactions, you'll have to scan_tx for them
• wallet-rpc is not persistent. I'm not sure how this behaves in practice. Example: change lookahead from default 200 to 500 -> receive tx on minor index 500 -> lookahead is now to index 1000 -> restart wallet = does the wallet forget about subaddress 701-1000? Or does it just not expand until it reaches 800 now?

future:

• wallet-rpc should be made persistent, perhaps in a follow-up pr.
• perhaps subaddress-lookahead should rollback the wallet height to the last known transaction, so missed transactions are found automatically

[plowsof]

wallet-rpc is not persistent.

Currently wallet rpc cache has to be saved manually with store, i dont know of anything which triggers store off the top of my head. *ctrl+c? 😄

[plowsof]

i've went down a rabbit hole sanity checking the effects of changing set subaddress-lookahead without this patch, these instructions will help us to test this PR #7364 (comment)