common: Add external annotations to Guava Cache and LoadingCache

common: Add external annotations to Guava Cache and LoadingCache

getIfPresent() should return a @Nullable V. Elsewhere it should
follow the declaration type.

Some other methods (asMap() etc.) are known to really return a
possibly empty map, so those can be marked @NonNull.

Change-Id: I5466151977ee639535b1f818ef9e3c14ba122f7e
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/85082
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI