Generates client-side, microsecond-precision query timestamps.
Given that Cassandra uses those timestamps to resolve conflicts, implementations should generate
monotonically increasing timestamps for successive invocations of next().
Returns the next timestamp.
Implementors should enforce increasing monotonicity of timestamps, that is,
a timestamp returned should always be strictly greater that any previously returned
timestamp.
Implementors should strive to achieve microsecond precision in the best possible way,
which is usually largely dependent on the underlying operating system's capabilities.
Returns:
the next timestamp (in microseconds). If it equals Long.MIN_VALUE, it won't be
sent by the driver, letting Cassandra generate a server-side timestamp.