Faster clear() method for RDS storrs and drivers

[wlandau]

This PR implements the original proposal in #121 (comment).

Changes

1. The RDS driver now has a clear() method, which uses unlink(recursive = TRUE) instead of deleting keys one by one from R.
2. The clear() method in the storr base class looks for a driver's clear() method first, then defaults to deleting keys one by one if the driver has no such method.
3. The return value of clear() in RDS drivers is integer(0) instead of the number of deleted keys.
4. Tests updated to reflect (3).

Benchmarks

We see the speed improvements we expected from #121 (comment).

library(storr)
init_storr <- function() {
  s <- storr_rds(tempfile())
  keys <- as.character(seq_len(2))
  values <- rep(TRUE, length(keys))
  s$mset(keys, values)
  s
}

s <- init_storr()
system.time(s$clear())
#>    user  system elapsed 
#>   0.000   0.000   0.001

^{Created on 2020-03-15 by the reprex package (v0.3.0)}