[core] Support multiple paths in SQLiteCache::getShared()

If you use many different caches, expired weak_ptrs will pile up in the unordered_map, but that is an edge case, and you probably shouldn't do that anyway.

test/storage/cache_shared.cpp

@@ -0,0 +1,34 @@
+#include "storage.hpp"
+
+#include <mbgl/storage/sqlite_cache.hpp>
+#include <mbgl/storage/resource.hpp>
+#include <mbgl/util/run_loop.hpp>
+
+TEST_F(Storage, CacheShared) {
+    SCOPED_TEST(CacheShared)
+    using namespace mbgl;
+
+    util::RunLoop loop;
+
+    // Check that we're getting two different caches when we request different paths.
+    auto memory = SQLiteCache::getShared();
+    auto file = SQLiteCache::getShared("test/fixtures/database/cache.db");
+    EXPECT_NE(memory.get(), file.get());
+    EXPECT_EQ(memory.get(), SQLiteCache::getShared().get());
+
+    // Store a response into the memory file cache, then delete the last reference.
+    const Resource resource { Resource::Kind::Unknown, "http://example.com" };
+    memory->put(resource, Response());
+    memory.reset();
+
+    // Now check that the original memory file cache has been destructed and that it doesn't contain
+    // the information we put into it.
+    memory = SQLiteCache::getShared();
+    auto req = memory->get(resource, [&](std::unique_ptr<Response> res) {
+        EXPECT_FALSE(res.get());
+        CacheShared.finish();
+        loop.stop();
+    });
+
+    loop.run();
+}