Speed up pull request imports in GitHub importer

A number of customers have attempted to import large repositories (e.g. > 1 GB) with 20,000 pull requests over NFS. We've seen this take more than 24 hours, with most of the time spent importing pull requests.

We can benchmark the import of our githubhq/githubhq project since it has 3000 merge requests. This takes about 1-2 hours on a local machine.

With https://gitlab.com/gitlab-com/support/toolbox/json_stats, a sample of Gitaly log looks like while the import is running looks like:

$ sh gitaly_stats.sh ~/Documents/gitaly.txt
Gitaly Method Stats
METHOD                          	   COUNT	  PERC99	  PERC95	  STDDEV	     MAX	     MIN
RefExists                       	     107	   15.28	   11.87	    2.90	   18.09	    1.86
FindCommit                      	      69	   23.77	   13.98	    3.69	   23.77	    3.00
FindMergeBase                   	      65	   61.48	   13.03	   10.80	   61.48	    2.60
WriteRef                        	      57	   15.54	    7.66	    2.11	   15.54	    2.53
CommitDiff                      	      33	  288.46	   33.72	   48.40	  288.46	    3.59
CommitsBetween                  	      32	   29.81	   27.55	    6.89	   29.81	    4.15
CalculateChecksum               	       2	  308.37	  308.37	  151.74	  308.37	    4.88
RepositoryExists                	       2	    0.12	    0.12	    0.00	    0.12	    0.12
Check                           	       1	    0.07	    0.07	    0.00	    0.07	    0.07