Add limited locking around the end of the call

[swankjesse]

This is mostly necessary because we have a possibility of duplex
calls.

[yschimke]

Test failures seem definitely related

okhttp3.URLConnectionTest > requestBodySurvivesRetriesWithFixedLength FAILED
    java.lang.IllegalStateException: cannot make a new request because the previous response is still open: please call response.close()
        at okhttp3.internal.connection.RealCall.enterNetworkInterceptorExchange(RealCall.kt:229)
        at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.kt:66)
        at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.kt:100)
        at okhttp3.internal.connection.RealCall.getResponseWithInterceptorChain$okhttp(RealCall.kt:201)
        at okhttp3.internal.connection.RealCall.execute(RealCall.kt:154)
        at okhttp3.URLConnectionTest.getResponse(URLConnectionTest.java:3763)
        at okhttp3.URLConnectionTest.testRequestBodySurvivesRetries(URLConnectionTest.java:311)
        at okhttp3.URLConnectionTest.requestBodySurvivesRetriesWithFixedLength(URLConnectionTest.java:288)
okhttp3.URLConnectionTest > disconnectDuringConnect_cookieJar FAILED
    java.lang.IllegalStateException: cannot make a new request because the previous response is still open: please call response.close()
        at okhttp3.internal.connection.RealCall.enterNetworkInterceptorExchange(RealCall.kt:229)
        at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.kt:66)
        at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.kt:100)
        at okhttp3.internal.connection.RealCall.getResponseWithInterceptorChain$okhttp(RealCall.kt:201)
        at okhttp3.internal.connection.RealCall.execute(RealCall.kt:154)
        at okhttp3.URLConnectionTest.disconnectDuringConnect_cookieJar(URLConnectionTest.java:1166)

[swankjesse]

Yep, failures were my fault. I move the assignments on line 259 up here to save a synchronized block, and that was broken if the lines in-between throw! Argh!

I believe the update doesn’t have behavior changes, other than introduced locking.