sstableloader: Handle dropped columns
sstableloader: Handle dropped columns

Fixes #1470

Origin sstable (legacy) parser is fine with finding column
that are not present in schema, as long as it can verify
this by checking the "droppedColumns" attribute of CF meta
data.
Since we build the metadata from the Java driver, we do
not have that info as such, but like the Java driver,
there is nothing preventing some old fashioned system
table queries.
This patch reads the (v3) system_schema.dropped_columns
table to find all dropped columns for a table and include
them in the metadata used by sstable reader.
It also adds an option -g/--ignore-missing-columns to 
explicitly set column names to treat as dropped in case
of either schema dropped info missing (recreated schemas),
or running against a v2 schema cluster. 
scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java