server: implement column filters in Select()

[dcbw]

Since the unit test for Monitor() requests all columns,
we now have to expect empty fields for those columns in
the response.

Previously Select() just ignored the columns that
Monitor() requested and thus elided empty columns from
the result.

@trozet @dave-tucker @jcaamano

[jcaamano]

Just a heads up on the different behavior of including empty columns whether they were requested or not. If no columns were requested, empty columns are not included. For requested columns, empty columns are included.

The RFC is not really explicit about it

   If "columns" is not specified, all the
   table's columns are included (including the internally generated
   "_uuid" and "_version" columns). 

Should we check what the ovs ovsdb server does?

[coveralls]

Pull Request Test Coverage Report for Build 1773018781

• 22 of 32 (68.75%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 73.399%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server/transact.go	2	3	66.67%
server/server.go	4	6	66.67%
ovsdb/set.go	1	4	25.0%
mapper/mapper.go	15	19	78.95%

Files with Coverage Reduction	New Missed Lines	%
server/transact.go	2	69.57%

Totals
Change from base Build 1754146115:	-0.03%
Covered Lines:	4423
Relevant Lines:	6026

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💛 - Coveralls