ENH: Archiver Timeplot address setters

[zdomke]

Made changes to the address setters for ArchivePlotCurveItem and TimePlotCurveItem classes:

1. Archive curve object now has an address setter that replaces the function setArchiveChannel. The setter calls the TimePlotCurveItem address setter, then sets the new address as the archive channel. This results in the live and archive channels always being tied to the same address.
2. Both address setters now also check that the new address is different from the current address. If they're the same, the function returns.
3. Changed the way the ArchivePlotCurveItem address setter replaces the address's protocol. Now the pydm utility is used.
4. The TimePlotCurveItem address setter clears all live data tied to the previous channel. Similarly, the ArchivePlotCurveItem removes all archived data for the previous channel.

[jbellister-slac]

Looks good, thanks!