laurenz/oracle_fdw ORACLE_FDW_2_8

Bugfixes

Throw an error if ctid or other system columns are used in the WHERE clause. Since these columns have no meaning in Oracle, they are assigned meaningless values, and using them in WHERE conditions would lead to meaningless results.
Reported on Stackoverflow.
Fix the server option nchar introduced in 2.4.0.
Due to an omission, that option never worked, and NCHAR and NVARCHAR2 columns were converted wrongly even if nchar was on.
Report and analysis by @chmyaf in #706.
Fix IMPORT FOREIGN SCHEMA so that it imports tables whose name is the same as the name of an index.
Report by Leho Lehes.
Don't push down LIMIT.
This is a regression from older releases, but the current implementation was incorrect, as reported by @gytune74 in #721.
Fix a crash with column options on non-existing columns.
If the Oracle table has fewer columns than the PostgreSQL foreign table and one of these extra columns has a column option set, oracle_fdw would crash.
Reported by @ganshinm, analysis by @ziva777 in #744.
Increase the size of the Oracle execution plan lines.
With the previous limit of 1000, EXPLAIN (VERBOSE) for some Oracle metadata queries resulted in
```
ORA-01406: fetched column value was truncated
```
Report by @ganshinm in #745 and @saygoodbye in #747.

Raise the limit for prefetch to 10240 again, like it was before version 2.6.0.
Requested by @nasmart in #671, @AmebaBrain in #674 and others.
Note that high values increase the risk of out-of-memory errors.
Add support for NCLOB.
Patch by @intgr in #563.