fix rare replication stream corruption with disk-based replication
The slave sends \n keepalive messages to the master while parsing the rdb, and later sends REPLCONF ACK once a second. rarely, the master recives both a linefeed char and a REPLCONF in the same read, \n*3\r\n$8\r\nREPLCONF\r\n... and it tries to trim two chars (\r\n) from the query buffer, trimming the '*' from *3\r\n$8\r\nREPLCONF\r\n... then the master tries to process a command starting with '3' and replies to the slave a bunch of -ERR and one +OK. although the slave silently ignores these (prints a log message), this corrupts the replication offset at the slave since the slave increases the replication offset, and the master did not. other than the fix in processInlineBuffer, i did several other improvments while hunting this very rare bug. - when redis replies with "unknown command" it includes a portion of the arguments, not just the command name. so it would be easier to understand what was recived, in my case, on the slave side, it was -ERR, but the "arguments" were the interesting part (containing info on the error). - about a year ago i added code in addReplyErrorLength to print the error to the log in case of a reply to master (since this string isn't actually trasmitted to the master), now changed that block to print a similar log message to indicate an error being sent from the master to the slave. note that the slave is marked as CLIENT_SLAVE only after PSYNC was received, so this will not cause any harm for REPLCONF, and will only indicate problems that are gonna corrupt the replication stream anyway. - two places were c->reply was emptied, and i wanted to reset sentlen this is a precaution (i did not actually see such a problem), since a non-zero sentlen will cause corruption to be transmitted on the socket.
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