R1:146fb646f92b

Automatically degrade to read-only mode when unable to connect to the master

Summary:
Ref T4571. If we fail to connect to the master, automatically try to degrade into a temporary read-only mode ("UNREACHABLE") for the remainder of the request, if possible.

If the request was something like "load the homepage", that'll work fine. If it was something like "submit a comment", there's nothing we can do and we just have to fail.

Detecting this condition imposes a performance penalty: every request checks…