Support larger cache sizes

[glightfoot]

The maxbytes parameter to New() is an int, which effectively limits the cache size to ~2GB. It would be nice if it supported int64 so we could create very large caches

[valyala]

int in Go is equivalent to int64 on 64-bit architectures. As for 32-bit archs, they usually cannot address more than 2^32 bytes in process memory.