Bump wallet version for pre split keypool

Summary:
Bump the wallet version to indicate support for the pre split keypool.
Also prevents any wallets from upgrading to versions between HD_SPLIT
and PRE_SPLIT_KEYPOOL.

Partial backport of Core PR12560 (4/4)
https://github.com/bitcoin/bitcoin/pull/12560/commits/a8da482a8bc87ff26194612727d4a7b86b2fb60d

Depends on D4206

Test Plan:
Using a wallet from before v0.19.7:

make check
./bitcoind -printtoconsole -upgradewallet=190700
./bitcoin-cli getwalletinfo

Above should output something like:

{
   "walletname": "",
   "walletversion": 200300,
   "balance": 0.00000000,
   "unconfirmed_balance": 0.00000000,
   "immature_balance": 0.00000000,
   "txcount": 0,
   "keypoololdest": 1570130795,
   "keypoolsize": 2000,
   "keypoolsize_hd_internal": 0,
   "paytxfee": 0.00000000,
   "hdmasterkeyid": "0a6b7ba8344e7f65abca305956e696c2b2d4023a"
}

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4207