bus: add API for querying the kdbus message sequence number
bus: simplify naming of feature negotation calls

Two verbs in a function name suck, so let's simplify this a bit.
bus: rename sd_bus_get_realtime_timestamp() to sd_bus_get_realtime_usec()

This brings the calls into similar style as the respective functions in
libsystemd-journal, and also is a bi shorter and more descriptive since
it clarifies the time unit used.