discovery: make gossip filter semaphore capacity configurable

In this commit, we make the gossip filter semaphore capacity configurable
through a new FilterConcurrency field. This change allows node operators
to tune the number of concurrent gossip filter applications based on
their node's resources and network position.

The previous hard-coded limit of 5 concurrent filter applications could
become a bottleneck when multiple peers attempt to synchronize
simultaneously. By making this value configurable via the new
gossip.filter-concurrency option, operators can increase this limit
for better performance on well-resourced nodes or maintain conservative
values on resource-constrained systems.

We keep the default value at 5 to maintain backward compatibility and
avoid unexpected resource usage increases for existing deployments. The
sample configuration file is updated to document this new option.

sample-lnd.conf

@@ -1778,6 +1778,12 @@
 ; maximum lightning message size (~65KB) to allow sending large gossip messages.
 ; gossip.msg-burst-bytes=2048000
 
+; The maximum number of concurrent gossip filter applications that can be
+; processed. Increase this value to handle more simultaneous peer
+; synchronizations at the cost of additional resource usage.
+; See docs/gossip_rate_limiting.md for mor information.
+; gossip.filter-concurrency=5
+
 [invoices]
 
 ; If a hold invoice has accepted htlcs that reach their expiry height and are