HTTP remote directory for tantivy. The directory is read-only, and is accessed on-demand by HTTP
range requests. The directory is backed by a large anonymous memory map, and pages are marked as
available to the kernel with MADV_FREE, meaning that the kernel can reclaim the memory if
needed. However, in situations of low memory pressure, previously fetched index pieces will
remain in memory for fast subsequent access. In practice, this means that several searches sent
in quick succession as a user is typing out a query will warm the cache for the final search,
making it feel faster.