<fix>[core]: cap client keep-alive below agent socket timeout

[ZStack-Robot]

MN's async RESTFacade HTTP client (Apache HttpAsyncClient) never set a
keep-alive strategy, so idle connections in the pool live forever: the
default strategy returns -1 when the agent sends no Keep-Alive header.
The agent cherrypy HTTP server closes idle keep-alive connections at its
socket_timeout (10s default, 15s after the agent-side fix). When the pool
reuses a connection the agent has already closed, the reused request hits
a RST and surfaces as "Connection reset by peer" wrapped in error 1015
"Cannot make a HTTP request", intermittently disconnecting hosts.

Cap the client keep-alive duration via a new global property
RESTFacade.keepAliveTimeMillis (default 5000ms, below the agent
socket_timeout) so the client always recycles an idle connection before
the agent closes it.

GlobalPropertyImpact

Resolves: ZSV-12612

This commit is cherry-picked from ZSTAC-86545
(commit: abe313c)

Change-Id: I70677a706d64637970766a64766d78726478776a

sync from gitlab !10535

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📥 Commits

Reviewing files that changed from the base of the PR and between 49a2020 and 6dd5bf6.

📒 Files selected for processing (3)

• core/src/main/java/org/zstack/core/CoreGlobalProperty.java
• core/src/main/java/org/zstack/core/rest/RESTFacadeImpl.java
• test/src/test/groovy/org/zstack/test/integration/core/rest/RestFacadeKeepAliveCase.groovy

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