Add index for common prediction query

Previously, requests to the `/v1/allocate` endpoint took around 57ms. After adding a composite index that covers the complete spec and the most common query during the prediction algo, we get a large speedup to around 1.3ms per request. While more indices could be added, the returns were diminished and more risky as we don't want to negatively impact the INSERT speed for the jobs table.

gantry/__main__.py

@@ -27,6 +27,7 @@ async def apply_migrations(db: aiosqlite.Connection):
         # they are applied in the correct order
         # and not inadvertently added to the migrations folder
         ("001_initial.sql", 1),
+        ("003_spec_index.sql", 3),
     ]
 
     # apply migrations that have not been applied

migrations/003_spec_index.sql

@@ -0,0 +1 @@
+CREATE INDEX complete_spec on jobs(pkg_name, pkg_variants, pkg_version, compiler_name, compiler_version, end);