smp-server: limit concurrent name resolutions

[shumvgolove]

RSLV is unauthenticated, and each RSLV runs an outbound HTTP/TLS request to the resolver. Nothing actually bounded these requests:

• forkCmd released its concurrency slot when the thread was forked, not when the command finished, so serverResolverConcurrency (and serverClientConcurrency for PFWD) limited nothing.
• There was no global limit: managerConnCount only sizes the idle pool, so excess requests opened new connections.
• Error responses were returned without reading the body, so http-client closed the connection and every 5xx cost a new TCP/TLS handshake.

Changes:

• forkCmd holds the slot until the command completes; the slot is also released if the fork fails.
• A global limit on concurrent resolver requests (new INI [NAMES] resolver_global_concurrency, default 32, 1-1000), counted within the resolver timeout; the connection pool is sized to it.
• Error response bodies are read (up to the response size limit), so the connection is reused.

Test	Before	After
16 RSLVs, per-connection limit 4	16 at the resolver	4
64 RSLVs, global limit 8	64 at the resolver	8
5 lookups answered with 502	5 connections	1

With the limit enforced, a connection that reaches it blocks its own command loop until a slot frees.