Currently SimpleCache will touch cache spans whenever it reads
from them. With legacy SimpleCache setups this involves a potentially
expensive file rename. With new SimpleCache setups it involves
a more efficient but still non-free database write.
For offline use cases, and more generally any use case where the
eviction policy doesn't use last access timestamps, touching is
not useful. This change allows the evictor to specify whether it
needs cache spans to be touched or not. SimpleCache will only touch
spans if the evictor requires it.
Note: There is a potential change in behavior in cases where a
cache uses an evictor that doesn't need cache spans to be touched,
but then later switches to an evictor that does. The new evictor
may temporarily make sub-optimal eviction decisions as a result.
I think this is a very fair trade-off, since this scenario is
unlikely to occur much, if at all, in practice, and even if it
does occur the result isn't that bad.
PiperOrigin-RevId: 244005682
afd72839dc