Release postmaster working memory context in slotsync worker

Release postmaster working memory context in slotsync worker

Child processes do not need the postmaster's working memory context and
normally release it at the start of their main entry point. However,
the slotsync worker forgot to do so.

This commit makes the slotsync worker release the postmaster's working
memory context at startup, preventing unintended use.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Tiancheng Ge <getiancheng_2012@163.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwHO05JaUpgKF8FBDmPdBUJsK22axRRcgmAUc2Jyi8OK8g@mail.gmail.com