Allows to disconnect all connections that are not currently in use. In particular, it allows to close all connections at shutdown to avoid leaking socket descriptors.
ConnectionPool!TCPConnection used to work with the old vibe-d:core implementation, but now failed, because TCPConnection is a struct. All assumptions about `Connection` being a class have been remove in this commit.
The library is able to support simple TCP servers in the current state. The API is still mostly compatible with mainline vibe.d, but the driver systen has been replaced by the eventcore library and sockets/files/timers/... are now structs with automatic reference counting instead of GC collected classes. The stream interfaces have been removed for now.
