_set_option (and set_water_level which uses it) are currently unsafe on firmwares below 2.1.9

[cmccambridge]

While chasing my tail on adding support to set and read back the time zone UTC offset in #63, I discovered that the _set_option method, as currently implemented, is only safe for firmwares 2.1.9 and above. The reason is: the syntax for the /co API had a breaking change in firmware 2.1.9:

2. Major Changes
   Compared to previous firmwares, some of the major changes in this firmware that affect the APIs include:
   ● Changing of option values (/co) now supports using JSON names. The previous way of using option index is no longer supported.

Prior to firmware 2.1.9, the /co Change Option API endpoint used a different syntax (e.g. 2.1.8 API manual):

• Instead of JSON variable names, each option was numbered with an index, like o1 instead of tz for Time Zone, or o23 instead of wl for Water Level.
• In those o?? firmwares, several options are highlighted in the API spec as boolean options which are set or cleared on every /co call, by whether or not they are present in the query string at all. (Value can be absent and is ignored... just presence = true, absence = false.)
  

So for the existing set_water_level routine: if we call that on a controller running older firmware than 2.1.9, not only will it not set the water level (since it is passing wl=<value> instead of o23=<value>, but 🐛 we'll actually be clearing options like o3 "Use DHCP"

So this issue is to either disable the _set_option routine for the older firmwares, OR, to build in proper support to use /co safely on those versions.