sql: use 'varbinary' as a name of type instead of 'blob'

We are going to introduce new column type 'VARBINARY', which allows to
store values with MP_BIN msgpack format. On the other hand, now it is also
possible to meet this type: all literals in form of x'...' are
supposed to be inserted into SCALAR column type exactly with MP_BIN
encoding. Prior to this moment type of such values (encoded as MP_BIN)
was called 'blob'. Thus, let's fix all visible to user messages using
'varbinary' name of type instead of 'blob'.