Update limit and offset

[pauldix]

This PR changes LIMIT and OFFSET to SLIMIT and SOFFSET. It also implements LIMIT and OFFSET to limit the number of data points returned per series in the result.

[toddboom]

This looks good. My only two questions are:

1. Should we use SLimit and SOffset instead of Slimit and Soffset for clarity?
2. Should the changes in server_test.go actually be duplicated/expanded so that we're testing both LIMIT/OFFSET and SLIMIT/SOFFSET?

[pauldix]

@toddboom: The tests for LIMIT and OFFSET are in server_integration_test.go, which is where these other ones should go eventually. I just didn't want to bother transferring them over in this PR.

Once the race condition stuff gets resolved so that writes don't have to wait 3 seconds in those integration tests, I'll get more aggressive about moving more of the tests in there.

Updated Slimit and Soffset to SLimit and SOffset in the code.

[toddboom]

rad, 👍!

[duylong]

Good feature +1

Could we not set a default limit ? (for example defined in the configuration file)