Add TTL option to Export

[b-gretsch]

For exports of big collections users may want to increase the cursor's TTL to a bigger value without having to change the server default.

[tomterl]

If new functionality and grammar/spelling corrections are done, I prefer multiple commits, separating the different types of change.

[frankmayer]

@b-gretsch Thanks for this PR.
I would normally agree with @tomterl on this one, however, since this is a really old PR that slipped through, I think it's OK to let this one through without going through the separation process.

However there are a few more steps to get this merged:

1. Provide a test which uses this added functionality
2. Resolve conflicts.
3. Provide the ArangoDB company with a signed CLA, see https://github.com/arangodb/arangodb-php/blob/devel/CONTRIBUTING.md

[frankmayer]

@b-gretsch are you still interested in completing this PR?

[frankmayer]

This PR seems abandoned. Leaving it open for consideration of implementing such functionality.