Get rid of common_timestamp function in 1.9.x

[SpacemanPaul]

The common_timestamp sql function is no longer required as it can be more efficiently replaced with the postgresql double colon (::) type-cast operator. This is not supported by SQLAlchemy 1.4, and so cannot be done in the 1.8 branch, but should be relatively easy to factor out in 1.9

(See opendatacube/datacube-explorer#595)