Wallet corruption while storing

[MrCyjaneK]

Steps to reproduce:

1. restore a wallet
2. start a refresh
3. while syncing call store() method
4. wallet should crash, if it didn't repeat step 3

expected results:

• wallet not crashing and wallet cache getting updated or
• wallet not crashing and store method being ignored or
• wallet crashing and cache file not getting corrupted

With the help of tobtoht I was able to figure a way around this bug by using the following patch:

https://git.mrcyjanek.net/mrcyjanek/monero_c/src/branch/rewrite-wip/patches/monero/0011-store-crash-fix.patch

it depends on the LOCK_REFRESH macro from background sync pr (https://github.com/monero-project/monero/pull/8617/files#diff-fcae134993cca2b6f1e887a60638f226ab00165e30bd4d326c17f0a58e11871fR57-R68)

also note that the issue exist in forked version of monero, with couple of patches applied, that's the reason behind this report being an issue and not a PR. If the patch looks about right I'll make this a PR

NOTE: technically you can still cause crash with this fix, when you try to modify the wallet state while storing.