out-line the SharedMutexImpl members that directly reference tls_lastTokenlessSlot

out-line the SharedMutexImpl members that directly reference tls_lastTokenlessSlot

Summary: Recent changes to SharedMutex.h broke mcrouter's open source build. See https://travis-ci.org/facebook/mcrouter/builds/140608809. It looks like we're getting bitten by https://sourceware.org/bugzilla/show_bug.cgi?id=16773. We're hitting it now because the extern template is forcing the instantiation of the thread-local static member to be located in SharedMutex.o, but the inline members that reference that thread-local are *not* located in SharedMutex.o. binutils seems to be stepping on its own feet trying to fix up the references at link time. We can fix it by making sure the code that references the thread-local is colocated with the thread-local.

Reviewed By: yfeldblum

Differential Revision: D3498477

fbshipit-source-id: 86ea86812010ff1ef7351e6f8c106bb4291d0234