Remove session_alloc (API Change)

* session_alloc is removed, as it does not improve
  performance as intended.

Actions Required:

* Don't use session_alloc

test/example/common/Jamfile

@@ -10,7 +10,6 @@
 local SOURCES =
     root_certificates.cpp
     server_certificate.cpp
-    session_alloc.cpp
     ;
 
 local RUN_TESTS ;