simplewallet: set subaddress-lookahead does not work after wallet creation

[benevanoff]

Description

While attempting to resolve #8954, I first tried to understand fully how it is implemented in the cli since woodser linked to the code snippet where it shows that there are class member variables m_subaddress_lookahead_major and m_subaddress_lookahead_minor that gets updated and saved to the wallet cache, but this made me realize that the m_subaddresses table of m_wallet is never actually updated and prompted me to test it, finding that it does not actually make the wallet "look ahead" any further.

User Impact

Users that have used many subaddresses may not be able to find their e-notes after restoring from their seed when adjusting the lookahead with set subaddress-lookahead

How to Reproduce

1. create a new wallet: wallet_1
2. run set - this will show the lookahead set to the default, 50:200
3. set subaddress-lookahead 50:300
4. run set again - this will now show 50:300
5. (optional) close and reopen the wallet then run set again to confirm the change has persisted
6. export the wallet seed and create a new wallet by restoring from the seed to create wallet_2
7. in wallet_2, run address new 300 times
8. run address all
9. copy one of the subaddresses from the 210-290 range
10. send some moneroj from a different wallet to that address from step 9.
11. wait until the transaction is confirmed
12. open and refresh wallet_1 and wallet_2
13. you will see a new transaction in wallet_2 but not wallet_1
14. run set in each wallet one more time to confirm that the subaddress-lookahead should be the same (they will both display 50:300) but they are in fact different, evidenced by the balance difference indicating that wallet_1's subaddress pubkey hashtable has not been properly updated to "lookahead" past the 200th subaddress to the 300th subaddress.

How to Fix

Modify wallet2::set_subaddress_lookahead to actually expand/shrink the m_subaddresses class member variable based on the input.