Handlers enhancements

[rawdaGastan]

Is your feature request related to a problem? Please describe

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Describe the solution you'd like

1. Use constants for magic numbers like 32 for PubKeySize and 2 * time.Second for MaxTimestampDelta.
2. Use one functions for returning errors and success response
3. repeated response msgs/errors should be a constant value
4. Longitude and Latitude should be float64 in Location struct
5. err == db.ErrRecordNotFound should use errors.Is
6. err.Error() == "version already set" should use errors.Is

[AlaaElattar]

-Longitude and Latitude should be float64 in Location struct is separated to another issue #77

[AlaaElattar]

Work Completed:

• applied all enhancements to registrar