* filer: keep .system internal folder in the default SQL table
Bucket-table SQL stores read the first path segment under /buckets as a
bucket name. The ListBuckets owner index lives at /buckets/.system/..., so
every write there hit isValidBucket(".system") == false and returned
"invalid bucket name .system", flooding the filer log on the postgres/mysql
backends. Route dot-prefixed internal folders to the default table by their
full path, like any other non-bucket entry.
* filer: keep .system internal folder in the default leveldb3 DB
Same guard as the SQL stores: leveldb3 would otherwise open a separate DB
for the .system owner-index folder instead of keeping it in the default DB.
* filer: keep .system internal folder under the default ydb prefix
Skips a DescribeTable round trip per operation on the .system owner-index
path, which never resolves to a real bucket table.
* filer: keep .system internal folder in the default arangodb collection
Avoids creating a stray collection for the .system owner-index folder.
Chris Lu