Stricter Thread<> interface

Implements a Thread<>::invoke interface that only permits calling
member functions with bound arguments.

Also introduce invokeWithResult, which accepts a callback and invokes
it, passing the result, on the invoking thread's RunLoop. This closes
the loop of two evented threads doing message passing.

platform/default/sqlite_cache_impl.hpp

@@ -2,7 +2,6 @@
 #define MBGL_STORAGE_DEFAULT_SQLITE_CACHE_IMPL
 
 #include <mbgl/storage/sqlite_cache.hpp>
-#include <mbgl/util/run_loop.hpp>
 
 namespace mapbox {
 namespace sqlite {
@@ -13,15 +12,12 @@ class Statement;
 
 namespace mbgl {
 
-class SQLiteCache::Impl : public util::RunLoop {
-    friend class util::Thread<SQLiteCache::Impl>;
-
+class SQLiteCache::Impl {
 public:
     Impl(const std::string &path = ":memory:");
     ~Impl();
 
-public:
-    void processGet(const Resource& resource, Callback callback);
+    std::unique_ptr<Response> processGet(const Resource&);
     void processPut(const Resource& resource, std::shared_ptr<const Response> response);
     void processRefresh(const Resource& resource, int64_t expires);
 
@@ -29,7 +25,6 @@ private:
     void createDatabase();
     void createSchema();
 
-private:
     const std::string path;
     std::unique_ptr<::mapbox::sqlite::Database> db;
     std::unique_ptr<::mapbox::sqlite::Statement> getStmt;