Add writable TimeToPurgeDeletes to ScanOptions

In HBase 1.x and before, it was possible to override all settings on the ScanInfo when overriding the flush and compaction scanner open coprocoessors. HBase 2.x changed the coprocessor model to instead allow changes to a limited set of options on the ScanOptions object.

In HBASE-19895 and HBASE-24321, we added support for KeepDeletedCells and MinVersions to be overriden by ScanOptions, as needed by Phoenix.

A 1.x coprocessor used at my day job overrides TimeToPurgeDeletes, and to convert it to HBase 2.x that property would need to be overridable from ScanOptions as well. This should be a straightforward extension of the previous work.