iks-server: timestamp sent with non-live samples values

+ value command sends sample timestamp together with sample value for all samples except live
+ timestamp is calculated as the current timestamp, truncated to correct sample timestamp
fixed error introduced in last commit
iks: get_latest_set command

+ returns latest sample timestamp amongst all params belonging to given set
Change way to generate json from params.xml

+ copy whats needed from xml description rather than remove unused
  sections, cause there are some undocumented tags
+ the only needed atributes for param are name, unit and short_name
iks: better error handling

+ don't crash if params.xml is invalid, print error to log and
  continue initialization
iks: update client about every average probe

+ now average probes works different than live probes
+ every average is sent even if its value hasn't changed
+ live probes works same as earlier
Fixed jump to latest date on amd64

* wxDatetime cannot properly handle time_t max value
on amd64.
Merge current stable version: 3.14.11
building unstable
building stable