Improve the merge

[fikovnik]

Ideally, there should be the following facing function:

# Merges dbs from `paths` into `db`.
#
#' @type (sxpdb | chr, chr[], lgl, lgl) -> sxpdb
merge_dbs <- function(db, paths, parallel, quiet = TRUE) { ... }

# Merges `source` into `target` return the remapping of all source indices into their new target indices as data frames.
#
# @type (sxpdb, sxpdb) -> data.frame<source_idx, target_idx>
merge_into <- function(target, source, quiet=TRUE) { ... }