Merge #19300: wallet: Handle concurrent wallet loading

9b009fae6e2eb0ab2ee7ce7882c3556a9ac363a7 qa: Test concurrent wallet loading (João Barbosa)
b9971ae5853c1d62e09d976a8705f4f731290d85 wallet: Handle concurrent wallet loading (João Barbosa)

Pull request description:

  This PR handles concurrent wallet loading.

  This can be tested by running in parallel the following script a couple of times:
  ```sh
  for i in {1..10}
  do
    src/bitcoin-cli -regtest loadwallet foo
    src/bitcoin-cli -regtest unloadwallet foo
  done
  ```

  Eventually the error occurs:
  ```
  error code: -4
  error message:
  Wallet already being loading.
  ```

  For reference, loading and already loaded wallet gives:
  ```
  error code: -4
  error message:
  Wallet file verification failed. Error loading wallet w1. Duplicate -wallet filename specified.
  ```

  Fixes #19232.

ACKs for top commit:
  MarcoFalke:
    Concept ACK 9b009fae6e2eb0ab2ee7ce7882c3556a9ac363a7 I have not reviewed the code
  hebasto:
    ACK 9b009fae6e2eb0ab2ee7ce7882c3556a9ac363a7, tested on Linux Mint 20 (x86_64):
  ryanofsky:
    Code review good-but-not-ideal ACK 9b009fae6e2eb0ab2ee7ce7882c3556a9ac363a7

Tree-SHA512: 0ccd77b03c0926e4c4e51efb31e193b93cb4b9ffe8bac6bb018f7344c55dfd939b873b8cf5e657dca73e6202eb75aa672de2acb787cc133184b0b3b51e47b972

0 files changed, 0 insertions, 0 deletions