amari.xlog: Emit config_get after every sync(attached) instead of only after every attach

We emit config_get after every attach from the beginning of xlog in
e0cc8a38 (amari.xlog: Initial draft). The reasoning here is that it is
useful by default to know configuration of a service.

In the previous patch we added sync events so that xlog stream becomes
self-synchronizing. To continue that line it is now useful to have that
config_get emitted after every such synchronization point instead of
only after attaching to the service. That's what hereby patch does.

As a bonus the code is reworked in a way that config_get setup is not
hardcoded anymore and config_get periodicity now can be controlled by
users via explicitly specifying config_get in the logspec.

amari/xlog.py

@@ -131,17 +131,26 @@ class LogSpec:
 def xlog(ctx, wsuri, logspecv):
     # make sure we always have meta.sync - either the caller specifies it
     # explicitly, or we add it automatically to come first with default
-    # 10x·longest periodicity.
+    # 10x·longest periodicity. Do the same about config_get - by default we
+    # want it to be present after every sync.
     lsync       = None
+    isync       = None
+    lconfig_get = None
     pmax = 1
     for (i,l) in enumerate(logspecv):
         pmax = max(pmax, l.period)
         if l.query == "meta.sync":
+            isync = i
             lsync = l
+        if l.query == "config_get":
+            lconfig_get = l
     logspecv = logspecv[:]  # keep caller's intact
     if lsync is None:
+        isync = 0
         lsync = LogSpec("meta.sync", [], pmax*10)
         logspecv.insert(0, lsync)
+    if lconfig_get is None:
+        logspecv.insert(isync+1, LogSpec("config_get", [], lsync.period))
 
 
     xl = _XLogger(wsuri, logspecv, lsync.period)
@@ -263,10 +272,6 @@ class _XLogger:
                 _, resp_raw = conn.req_(ctx, query, opts)
             return resp_raw
 
-        # emit config_get after attach
-        cfg_raw = req_(ctx, 'config_get', {})
-        xl.emit(cfg_raw)
-
         # loop emitting requested logspecs
         t0 = time.now()
         tnextv = [0]*len(xl.logspecv)   # [i] - next time to arm for logspecv[i] relative to t0