Populate timestamps with microsecond precision

xyz.openbmc_project.Time.EpochTime is defined as time elapsed since the
epoch in microseconds.

xyz.openbmc_project.Common.Progress.StartTime and
xyz.openbmc_project.Common.Progress.CompletedTime are similarly
defined as microseconds.

Change timestamps that are currently captured in seconds to
microseconds.

Tested:

Check xyz.openbmc_project.Time.EpochTime,
xyz.openbmc_project.Common.Progress.StartTime, and
xyz.openbmc_project.Common.Progress.CompletedTime timestamps of dump
entries.

Example:
busctl introspect xyz.openbmc_project.Dump.Manager /xyz/openbmc_project/dump/bmc/entry/1

Signed-off-by: Claire Weinan <cweinan@google.com>
Change-Id: I6ea220af07f06ad79de0b62b24ce6cd5ba5fd33d