SERVER-6656 prevent SSL memory leaks

If a BackgroundTask should attempt to use SSL (via an outbound connection),
this commit will now ensure the SSL thread-specific data is cleaned up on
thread exit.
SERVER-8130 Fix invalid access in textSearchEnabled check

Bug introduced in fts_index.cpp, FTSIndexPlugin::adjustIndexSpec()
(not changed by diff):

 69|            StringData desc  = cc().desc();

Previously, Client::desc() returned std::string, so desc._data
became a dangling pointer to destroyed temporary cc().desc().
Resolved by changing return type of Client::desc() to StringData.
SERVER-8139 add special flag to minvalid during initial sync
SERVER-8139 cleanup

'h' is not needed in minValid recorded in the database; it is never
read.
The dummy minValid value is no longer needed either.