ENT-14195 1. Refactored cf-reactor event handling with one unified interface

[victormlg]

Rather than exposing the raw file-descriptor bookkeeping required for select(2), we introduce a unified interface that serves both the reactor-plugin and event-driven code paths. This is achieved by encapsulating all relevant state in a context struct, ReactorContext:

typedef struct ReactorContext
{
    Seq *fds // array of ReactorFd, which holds the fd and some metadata
    fd_set readfds;
} ReactorContext;

• ReactorContextInitialize(): initializes the reactor-plugin and event-driven code. Wraps ReactorNovaInitialize()
• ReactorContextSetupFileDescriptors(): populates readfds with the file descriptors to monitor, prior to the select() call.
• ReactorContextHandleEvents(): iterates over the file descriptors and dispatches the appropriate action based on which ones were signaled as ready. Wraps ReactorNovaHandleTimeout and ReactorNovaHandleEvents().
• ReactorContextFinalize(): releases the daemon's associated resources. Wraps ReactorNovaFinalize().

[larsewi]

What if we stored FDs in a dynamic list instead (e.g., Seq)? Furthermore, maybe the FD itself could be wrapped in a struct with flags to tell what type of FD it is, and whether or not it is owned by any external plugin.