gss code cleanup

some things still need to be addressed:

• 1. rename gss_tsig_keys_mgr.h/.cc to managed_key.h/.cc
• 2. TKeyExchange can store the IOService on constructor instead of the doExchange method
• 3. check the key's security context on TKeyExchange constructor and throw if it has already been used
• 4. add core documentation for expiration-timeout and a table explaining all parameters
• 5. add core documentation about keys being deleted if more than 3 times maximum lifetime