42fb5311b1 rpc: return warnings as an array instead of just a single one (stickies-v)
Pull request description:
The RPC documentation for `getblockchaininfo`, `getmininginfo` and `getnetworkinfo` states that "warnings" returns "any network and blockchain warnings". In practice, only a single warning (i.e. the latest one that is set) is returned, the other ones are ignored.
Fix that by returning all warnings as an array.
As a side benefit, clean up the GetWarnings() logic.
Since this PR changes the RPC result schema, I've added release notes. Users can temporarily revert to the old results by using `-deprecatedrpc=warnings`, until it's removed in a future version.
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Some historical context from git log:
- when `GetWarnings` was introduced in 401926283a, it was used in the `getinfo` RPC, where only a [single error/warning was returned](401926283a (diff-7442c48d42cd5455a79915a0f00cce5e13359db46437a32b812876edb0a5ccddR250)) (similar to how it is now).
- later on, "warnings" RPC response fields were introduced, e.g. in ef2a3de25c, with the description [stating](ef2a3de25c (diff-1021bd3c74415ad9719bd764ad6ca35af5dfb33b1cd863c0be49bdf52518af54R411)) that it returned "any network warnings" but in practice still only a single warning was returned
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