Set the GRPC deadline and DBConnection checkout timeout to use :timeout

[emhagman]

@ospaarmann knows about this. We were not setting the GRPC deadline so a query that took too long past the 15 second DBConnection timeout would continue to run on Dgraph. Separately, DBConnection checkout was never being set so the connection would always close after 15 seconds but would retry the query up until the :timeout value set. This PR fixes both of those issues.

[coveralls]

Pull Request Test Coverage Report for Build 148

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 68.936%

---

Totals
Change from base Build 137:	0.4%
Covered Lines:	162
Relevant Lines:	235

---

💛 - Coveralls