Support AWS Lambda

[emanuelez]

AWS Lambda only allows writing to a folder which is mounted with a file system that does not support posix_fallocate. This means that Realm will fail and throw an exception:

Error: posix_fallocate() failed: Operation not permitted

which comes from here: https://github.com/realm/realm-core/blob/master/src/realm/util/file.cpp#L822

Basically, the problem is that we take a decision at compile time, when only at runtime we really have all the elements to take it.

I think that, before throwing that exception, we should try falling back to the non-atomic version (available here: https://github.com/realm/realm-core/blob/master/src/realm/util/file.cpp#L690) if posix_fallocate returns EINVAL.

[realm-probot]

Hey - looks like you forgot to add a T:* label - could you please add one?