use nanoTime instead of wall clock time to guarantee monotonicity

[gurghet]

The reason is nanoTime gives you a monotonic value (representing the number of nanoseconds since system boot) while instant gives you a non-monotonic value (millis since epoch). The latter is subject to changes made to the system clock (including it going backwards), usually as result of NTP adjustments. See also https://medium.com/@kralka/its-about-time-d42e68dbe1df

Originally posted by @sirocchj in #212 (comment)

[sirocchj]

This is now fixed as of v0.9.0.

See

tamer/db/src/main/scala/tamer/db/model.scala

Lines 11 to 12 in 8613425

	final case class ChunkWithMetadata[V](chunk: Chunk[V], pulledAt: Long = System.nanoTime())
	final case class ValueWithMetadata[V](value: V, pulledAt: Long = System.nanoTime())

and

tamer/db/src/main/scala/tamer/db/DbSetup.scala

Lines 58 to 60 in 8613425

	start <- UIO(System.nanoTime())
	(values, time) <- process(query, chunkSize, transactor, queue, currentState)
	newState <- stateFold(currentState, QueryResult(ResultMetadata(time - start), values.map(_.value)))