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.

lncfg/gossip.go

@@ -37,9 +37,12 @@ type Gossip struct {
 	MsgRateBytes uint64 `long:"msg-rate-bytes" description:"The total rate of outbound gossip messages, expressed in bytes per second. This setting controls the long-term average speed of gossip traffic sent from your node. The rate limit is applied globally across all peers, not per-peer. If the rate of outgoing messages exceeds this value, lnd will start to queue and delay messages to stay within the limit."`
 
 	MsgBurstBytes uint64 `long:"msg-burst-bytes" description:"The maximum burst of outbound gossip data, in bytes, that can be sent at once. This works in conjunction with gossip.msg-rate-bytes as part of a token bucket rate-limiting scheme. This value represents the size of the token bucket. It allows for short, high-speed bursts of traffic, with the long-term rate controlled by gossip.msg-rate-bytes. This value must be larger than the maximum lightning message size (~65KB) to allow sending large gossip messages."`
+
+	FilterConcurrency int `long:"filter-concurrency" description:"The maximum number of concurrent gossip filter applications that can be processed. If not set, defaults to 5."`
 }
 
 // Parse the pubkeys for the pinned syncers.
+
 func (g *Gossip) Parse() error {
 	pinnedSyncers := make(discovery.PinnedSyncers)
 	for _, pubkeyStr := range g.PinnedSyncersRaw {