dev-libs/leveldb: bump to 1.20, sub-slot to avoid ABI breakage

dev-libs/leveldb: bump to 1.20, sub-slot to avoid ABI breakage

* Upstream broke the ABI without changing the SONAME so we have to
  introduce a sub-slot, see
  https://github.com/google/leveldb/issues/536
  All reverse dependencies should now have the slot operator set, which
  will trigger rebuilds when required. The case of bitcoin is special
  because they only support specific leveldb versions.
* Properly handle USE=static-libs when USE=snappy isn't set, change
  package.use.mask entry. See https://bugs.gentoo.org/651604
* Update the patch to build libmemenv.so which is required by bitcoin,
  see https://github.com/google/leveldb/issues/236
* The patch also disables test building, allowing us to make src_compile
  more elegant.
* Remove MIPS patch which was merged: https://github.com/google/leveldb/pull/272
* Bump to EAPI=7

Closes: https://bugs.gentoo.org/609966
Package-Manager: Portage-2.3.40, Repoman-2.3.9