grpc/dircacheserver: forgot to RLock when reading the log at startup.
grpc/dircacheserver: forgot to RLock when reading the log at startup.

The lock doesn't matter except that the code tries to release it and
reobtain it as a write lock when adding an access file. The result was
trying to RUnlock something that wasn't RLocked.

Change-Id: I16e00682afe3ecc0276ce018cf05f88d146a4a98
Reviewed-on: https://upspin-review.googlesource.com/6560
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>
Reviewed-by: Eric Grosse <f6ae9ca1078fda92fd7c2c20411c29bc6711159e@google.com>

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