Shade Guava in google-cloud-spanner

[saturnism]

Spanner.closeAsync() exposes ListenableFuture from Guava

Causing issues when writing a POC w/ @emmanuelbernard on Hibernate OGM dialect.

[garrettjonesgoogle]

This should be switched to ApiFuture from GAX.

[vkedia]

@garrettjonesgoogle I started looking into this. There does not seem to be any way to convert a ListenableFuture to ApiFuture. In absense of that change becomes more involved since this future internally waits on completion of a list of ListenableFutures for which I am using Futures.allAsList but there is no such method for ApiFutures.
Do you think it makes sense to expose a method in gax that converts a ListenableFuture to ApiFuture? It does seem to exist internally:
https://github.com/googleapis/gax-java/blob/master/src/main/java/com/google/api/gax/core/internal/ApiFutureToListenableFuture.java

[vkedia]

Sent your a PR to add allAsList to ApiFutures
googleapis/gax-java#267

[garrettjonesgoogle]

If gax exposed a method that converted a ListenableFuture to ApiFuture, the whole point of creating ApiFuture to wrap ListenableFuture would be defeated. I think adding allAsList to ApiFutures is the right way to go.

[vkedia]

I believe this is done. Though I also sent a #2026 which removes the need for future completely by making close a blocking call.

[vkedia]

Closed by #1960