a9b6ba0b7 Add missing cs_main locks when calling blockToJSON/blockheaderToJSON (practicalswift)
Pull request description:
`blockToJSON(...)` and `blockheaderToJSON(...)` read the variable `chainActive` which requires holding the mutex `cs_main`. So does `GetDifficulty(...)`.
Tree-SHA512: bfb94f5e3238accbf6a4daddde49d53f1891c38ae9b07e25b3098c485747159258f64bb66a50e147b32beac601de89d9d04ff717b6c4f1460d329c90a53d3333
Backport of Core PR 11618
https://github.com/bitcoin/bitcoin/pull/11618/files