🐛 Fakeclient: Fix dataraces when writing to the scheme #3143
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We have a scheme write lock but plenty of other codeptaths that read from the scheme and that don't do looking, resulting in dataraces if the two happen in parallel.
This change introduces a simple RW lock and makes the fakeclient acquire read locking for all its operations except when needing the write lock. This isn't particularly smart, but given that we only have one codepath that writes to the scheme, it seems good enough.
Fixes #3139
/cherrypick release-0.20
/cherrypick release-0.19