[CLI feature request] USE retention policy for queries

[beckettsean]

Similar to USE mydb but USE retention_policy. Useful if I want to repeatedly query measurements in a non-default retention policy.

Not sure about the syntax. Maybe USE RP <rp> or USE_RP <rp>.

[beckettsean]

@corylanou there appears to be a feature like this already, or maybe it's related to the work @DanielMorsing did on INTO queries:

> show retention policies on mydb
name    duration    replicaN    default
raw 2057h36m0s  1       true
foo 2016h0m0s   1       false

> insert into foo boo value=42
Using retention policy foo

> select * from foo.boo
name: boo
---------
time                value
2015-11-04T21:30:56.791303649Z  42

> select * from raw.boo

> insert boo value=12
> select * from raw.boo
> select * from foo.boo
name: boo
---------
time                value
2015-11-04T21:30:56.791303649Z  42
2015-11-04T21:33:03.781864143Z  12

> insert into raw
ERR: Post http://localhost:8086/write?consistency=any&db=mydb&precision=n&rp=foo: EOF

> insert into raw boo value=67
Using retention policy raw
> select * from foo.boo
name: boo
---------
time                value
2015-11-04T21:30:56.791303649Z  42
2015-11-04T21:33:03.781864143Z  12

> select * from raw.boo
name: boo
---------
time                value
2015-11-04T21:33:27.823305637Z  67