Unify ErrorCodes COMMAND_NOT_IMPLEMENTED and NO_COMMAND_MATCHED into COMMAND_UNKNOWN

[tatu-at-datastax]

(alternative: unify as existing COMMAND_NOT_IMPLEMENTED -- I like COMMAND_ as prefix)
(another alternative: COMMAND_UNRECOGNIZED)

Currently there are 2 separate ErrorCodes used essentially for the same issue: case of API request referring to Command that we do not recognized (unknown, unmapped, mis-spelled).
Neither name is quite right:

• COMMAND_NOT_IMPLEMENTED -- this leads to assumption that such command might be added in future, which may or (more likely) may not be the case
• NO_COMMAND_MATCHED -- slightly better, but should start with COMMAND. Also, not sure "matching" conveys exact meaning.

Neither response is widely used: for past 48 hours there are not a single instance of either (out of ~5,000 error responses) so renaming should be safe to do at this point.

EDIT: Actually COMMAND_NOT_IMPLEMENTED is an internal failure for known command without matching resolver. Needs to be changed to new SERVER_INTERNAL_ERROR