Support for multiple-schemas

[shlomi-noach]

Based on #45 by @AnshumanTripathi, and with much gratitude, this PR introduces multi-schema support.

It is now possible to query multiple schemas per host.

To maintain backwards compatibility a new flag is added:

  -s string
      List of databases to query from; overrides -d, prints schema name to output

• When -s is provided, there is no need for -d.
• When -s is provided, the schema name is listed as 2nd column in output

Example:

$ echo $hosts | ccql -t 0.5 -s test -q "select uuid() from dual" | column -t
host3:3306    test  d0d95311-b8ad-11e7-81e7-008cfa542442
host2:3306    test  d0d95fd6-b8ad-11e7-9a23-008cfa544064
host1:3306    test  d0d96924-b8ad-11e7-9bde-008cfa5440e4
host4:3306    test  d0d99a9d-b8ad-11e7-a680-008cfa542c9e

$ echo $hosts | ccql -t 0.5 -s test,meta -q "select uuid() from dual" | column -t
host3:3306    test  d0d95311-b8ad-11e7-81e7-008cfa542442
host2:3306    meta  d0d95311-b8ad-11e7-a16c-a0369fb3dc94
host2:3306    test  d0d95fd6-b8ad-11e7-9a23-008cfa544064
host1:3306    meta  d0d95311-b8ad-11e7-9a15-a0369fb5fdd0
host3:3306    meta  d0d95311-b8ad-11e7-bd26-a0369fb5f3d8
host4:3306    meta  d0d95311-b8ad-11e7-a16c-a0369fb3dc94
host1:3306    test  d0d96924-b8ad-11e7-9bde-008cfa5440e4
host4:3306    test  d0d99a9d-b8ad-11e7-a680-008cfa542c9e

[shlomi-noach]

@AnshumanTripathi makes sense?

[AnshumanTripathi]

👍