Use custom isc_mem based allocator for libuv

Use custom isc_mem based allocator for libuv

The libuv library provides a way to replace the default allocator with
user supplied allocator (malloc, realloc, calloc and free).

Create a memory context specifically for libuv to allow tracking the
memory usage that has originated from within libuv.  This requires
libuv >= 1.38.0 which provides uv_library_shutdown() function that
assures no more allocations will be made.