Merge #11618: rpc: Lock cs_main in blockToJSON/blockheaderToJSON

Summary:
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

Test Plan:
make check
test_runner.py
bitcoin-cli getblock 000000000933ea01ad0ee984209779baaec3ced90fa3f408719526f8d77f4943

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2684