CXXCBC-404: KV_LOCKED status should be exposed as cas_mismatch for unlock

[DemetrisChr]

KV_LOCKED should be exposed as cas_mismatch only for the unlock operation. This makes it consistent with the behaviour of other SDKs and the expectation of the relevant FIT test.

From the retry handling RFC:

Locked 0x09 (Reason: KV_LOCKED)
Note: do NOT retry on KV_LOCKED for the "Unlock" command (the retry handler should never see it in the first place! (Reason is that this needs to be surfaced as a cas mismatch exception to the user and not being retried