be8ab7d08 Create new wallet databases as directories rather than files (Russell Yanofsky)
26c06f24e Allow wallet files not in -walletdir directory (Russell Yanofsky)
d8a99f65e Allow wallet files in multiple directories (Russell Yanofsky)
Pull request description:
This change consists of three commits:
* The first commit is a pure refactoring that removes the restriction that two wallets can only be opened at the same time if they are contained in the same directory.
* The second commit removes the restriction that `-wallet` filenames can only refer to files in the `-walletdir` directory.
* The third commit makes second commit a little safer by changing bitcoin to create wallet databases as directories rather than files, so they can be safely backed up.
All three commits should be straightforward:
* The first commit adds around 20 lines of new code and then updates a bunch of function signatures (generally updating them to take plain fs::path parameters, instead of combinations of strings, fs::paths, and objects like CDBEnv and CWalletDBWrapper).
* The second commit removes two `-wallet` filename checks and adds some test cases to the multiwallet unit test.
* The third commit just changes the mapping from specified wallet paths to bdb environment & data paths.
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**Note:** For anybody looking at this PR for the first time, I think you can skip the comments before _20 Nov_ and start reading at https://github.com/bitcoin/bitcoin/pull/11687#issuecomment-345625565. Comments before _20 Nov_ were about an earlier version of the PR that didn't include the third commit, and then confusion from not seeing the first commit.
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