Fetch plugin data concurrently to improve performance

Add an option to configure the number of threads to use for fetching
data from the Gerrit REST API concurrently. By default use 2 threads.
I could create the plugin page in less than a minute using this default.

Using more threads may lead to 429 errors by exceeding REST API rate
limits. Your mileage may vary depending on rate limits and recent
history of requests you have executed.

Change-Id: Id59e22c75baaeff8cf122094ad91cb6552ee67a6
1 file changed