Partial backport of core PR10740
Commit 5d15260 Add loadwallet RPC
https://github.com/bitcoin/bitcoin/pull/10740/commits/5d152601e940bd5f4043253b216a645679aff75d
The new `loadwallet` RPC method allows an existing wallet to be loaded dynamically at runtime. `unloadwallet` and `createwallet` are not implemented. Notably, `loadwallet` can only be used to load existing wallets, not to create a new wallet.
Commit a46aeb6 Test loadwallet
https://github.com/bitcoin/bitcoin/pull/10740/commits/a46aeb690141f8457cf7d19f5b4b84f97ce410c8
Add testcases to wallet_multiwallet.py to test the new `loadwallet` RPC method.
Commit cd53981 Add release notes for loadwallet RPC.
https://github.com/bitcoin/bitcoin/pull/10740/commits/cd53981b3d0d4697ed46c7bedbf10f464aca4ccc
Completes T459
Depends on D3635