add "turn cache off" function

[mpadge]

dodgr_cache_off <- function () {
    Sys.setenv ("DODGR_CACHE" = "FALSE")
}
is_dodgr_cache_on <- function () {
    as.logical (Sys.getenv ("DODGR_CACHE", unset = "TRUE"))
}
is_dodgr_cache_on ()
#> [1] TRUE
dodgr_cache_off ()
is_dodgr_cache_on ()
#> [1] FALSE

^{Created on 2019-06-05 by the reprex package (v0.3.0)}

This is necessary for other packages which need to just call one or two functions and return results as quickly as possible. With full caching, any additional function calls wait until the whole background process has finished before proceeding, so there really needs to be a way to stop that.

[mpadge]

see njtierney/maxcovr/issues/77 for why this function is necessary.