Sync against taskchampion-sync-server (#3118)

This removes use of gnutls and the TLS implementation, which is no
longer needed (task synchronization is handled via Taskchampion, which
uses `reqwest`, which handles TLS via other Rust dependencies). This
incidentally removes the following config options:
 * `debug.tls`
 * `taskd.ca`
 * `taskd.certificate`
 * `taskd.ciphers`
 * `taskd.credentials`
 * `taskd.key`
 * `taskd.server`
 * `taskd.trust`

src/tc/CMakeLists.txt

@@ -10,6 +10,7 @@ set (tc_SRCS
   ffi.h
   util.cpp util.h
   Replica.cpp Replica.h
+  Server.cpp Server.h
   WorkingSet.cpp WorkingSet.h
   Task.cpp Task.h)