Implement ThreadedSocketAcceptor.IsLoggedOn
.
Use linq instead of manual iteration
Merge branch 'ThreadedAcceptor_IsLoggedOn' of github.com:RFQ-hub/quickfixn into isloggedon_220
(#220) preserve existing interface, make same change to IInitiator
credits for #220/#235
Merge pull request #235 from connamara/isloggedon_220

IIinitiator/IAcceptor.IsLoggedOn method changed to property (includes #220)