R6 $finalize() methods

[wlandau]

For memory efficiency.

[wlandau]

...especially with #19.

[wlandau]

Should be private: https://r6.r-lib.org/articles/Introduction.html#finalizers.

[wlandau]

Actually, R is smart enough to free memory on garbage collection even when we go heavy on reference semantics. We will need a finalizer for the metadata database, but probably not in other situations.

> mem_used()
101 MB
> x <- runif(1e8)
> mem_used()
901 MB
> a <- init_target()
> a$get_cache()$set_value("x", x)
> b <- init_target()
> b$get_cache()$set_value("x", x)
> rm(x)
> gc()
            used  (Mb) gc trigger   (Mb)  max used
Ncells   1369770  73.2    2067778  110.5   2067778
Vcells 103072729 786.4  151019151 1152.2 103222981
        (Mb)
Ncells 110.5
Vcells 787.6
> mem_used()
901 MB
> rm(a)
> mem_used()
901 MB
> gc()
            used  (Mb) gc trigger   (Mb)  max used
Ncells   1369533  73.2    2067778  110.5   2067778
Vcells 103072906 786.4  151019151 1152.2 103222981
        (Mb)
Ncells 110.5
Vcells 787.6
> mem_used()
901 MB
> rm(b)
> gc()
          used (Mb) gc trigger  (Mb)  max used  (Mb)
Ncells 1369203 73.2    2067778 110.5   2067778 110.5
Vcells 3072857 23.5  120815321 921.8 103222981 787.6
> mem_used()
101 MB
>