HTTP module bug fix: 'query' part was missed in the http request first line

http: request first line should contain 'path' and 'query' parts
log: avoid streamsize to size_t warning
support 'timestamp' parameter for a "command/poll" request

update cloud6::Command to support 'timestamp' property (UTC date and time as simple string)
update simple_dev/simple_gw/zigbee_gw examples (REST) to support the last received command timestamp

NOTE: this 'timestamp' is valid during application lifetime only, to be able to process "offline" commands (which are received by server while application is not running) this 'timestamp' parameter should be saved between application runs.