Check database existence before connecting

[schedutron]

For #1164. @pmenon Can you please add the in progress label? Also, to check for database existence in traffic_cop.h, we need access to the Catalog object, right? Can you please hint me on how do we do that?

[coveralls]

Coverage decreased (-0.007%) to 76.852% when pulling 2afd501d57c98aba4a9b8e8d653215136adb7d06 on schedutron:dbconnect into 4f1bf3e9ff8072e560d53cc893219c2f1d6c1e25 on cmu-db:master.

[schedutron]

@pmenon Can you please hint me on how do I accomplish this check? I basically need to check for membership in a set of available databases' names.

[apavlo]

@ChTimTsubasa Can you help @schedutron with this?

[ChTimTsubasa]

The place you might want to insert the code to check the database existence is in https://github.com/cmu-db/peloton/blob/master/src/network/postgres_protocol_handler.cpp#L1118, where the protocol handler is checking the existence of the packet and send back a response. You might want to check what would be the correct response to send back. Using Wireshark on psql and Postgres would help.

[eribeiro]

Related to #1164