18.2.0 More Pushdown capabilities plus fixes

What's Changed

• db2_fdw 18.2.0 Enable push down functions by @temuenz in #116
• #117 initialize sqllen and null indicators before each fetch by @temuenz in #118
• Db2 fdw percentile push down by @temuenz in #120
• Fix schema-qualification of derived-table pushdown queries by @temuenz in #123
• Fix issue 121 clob encoding corruption by @temuenz in #124
• Remove nls_lang: inert Oracle NLS_LANG cruft inherited from oracle_fdw by @temuenz in #125

Full Changelog: 18.1.2...18.2.0