CCBC-505: Handle finalized_error_ctx when ctx is NULL

The code was written on the assumption that a socket error is always
invoked from within the socket handler callback, and therefor the
context will never be null. With lcb_node_chstate we artificially invoke
a socket error, even if the socket is already gone.

Change-Id: Id4ce4b2e3a55b8643431807044d80678899f2e89
CCBC-505: Add additional comments about chstate

Change-Id: I2fee4b4f35636e6c78efb1119a85aa87da40816c