make next_session_id thread safe

[fingera]

Make next_session_id thread safe

next_session_id called from session constructor

Main thread:

eos/plugins/bnet_plugin/bnet_plugin.cpp

Line 1262 in f9a3d02

auto s = std::make_shared<session>( *_ioc, shared_from_this() );

eos/plugins/bnet_plugin/bnet_plugin.cpp

Line 1429 in f9a3d02

auto s = std::make_shared<session>( ioc, my );

Work thread(on_accept):

eos/plugins/bnet_plugin/bnet_plugin.cpp

Line 1293 in f9a3d02

newsession = std::make_shared<session>( move( _socket ), _net_plugin );