SSTables loaded to the system via /upload dir may sometimes be needed
to generate view updates from them (if their table has accompanying
views).
Fixes#4047
Previous implementation assumes that each SSTable resides directly
in table::datadir directory, while what should actually be used
is directory path from SSTable descriptor.
This patch prevents a regression when adding staging sstables support
for upload/ dir.
distributed_loader is a sizeable fraction of database.cc, so moving it
out reduces compile time and improves readability.
Message-Id: <20181230200926.15074-1-avi@scylladb.com>
