Add semaphore to error store when flushing cached reports

[fractalwrench]

Adds a semaphore to the ErrorStore that is used when flushing cached reports. This prevents a scenario where a report could potentially be sent multiple times.

[Cawllec]

Referring to a desk conversation: This assertion was failing inconsistently when the emulator was first launched with the error:

java.lang.AssertionError: expected:<1> but was:<0>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:834)
at org.junit.Assert.assertEquals(Assert.java:645)
at org.junit.Assert.assertEquals(Assert.java:631)
at com.bugsnag.android.ErrorStoreAsyncTest.testLaunchThenConnectivity(ErrorStoreAsyncTest.java:90)
at java.lang.reflect.Method.invoke(Native Method)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)

[fractalwrench]

Unit test removed as this will be covered by mazerunner instead.

[coveralls]

Pull Request Test Coverage Report for Build 1245

• 11 of 13 (84.62%) changed or added relevant lines in 1 file are covered.
• 14 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.4%) to 70.748%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdk/src/main/java/com/bugsnag/android/ErrorStore.java	11	13	84.62%

Files with Coverage Reduction	New Missed Lines	%
sdk/src/main/java/com/bugsnag/android/ErrorStore.java	1	61.04%
sdk/src/main/java/com/bugsnag/android/SessionTracker.java	13	55.64%

Totals
Change from base Build 1217:	-0.4%
Covered Lines:	1768
Relevant Lines:	2499

---

💛 - Coveralls

[fractalwrench]

@kattrali I have run the 3 new mazerunner scenarios against this and they pass, whereas on master they would fail.

[kattrali]

Confirmed this works in an environment with slow network speeds and high latency.