folly::EventBase: wrap libevent calls to prevent race-condition

folly::EventBase: wrap libevent calls to prevent race-condition

Summary:
Patch D1585087 exposes two flaws in EventBase().  It introduces IO
worker threads to the ThriftServer which are constructed/destructed in
parallel.

Within the construction phase, a new EventBase() is instantiated for
each thread and unwound in destruction.

When using the BaseControllerTask (in Python), the following sequence
is observed:

a = event_init() [ThriftServer]
b = event_init() [IO worker 1]
c = event_init() [IO worker 2]
...
event_base_free(c)
event_base_free(b)
event_base_free(a)  -> segfault

1. event_init() should only ever be called once.  It internally
modifies a global variable in libevent, current_base to match the
return value.  event_base_free() will set current_base back to NULL if
the passed in arg matches current_base.   Therefore subsequent calls
must use event_base_new().

2. Since current_base is a global and EventBase() is called by multiple
threads, it is important to guard with a mutex.  The guard itself also
exposed the bug because:

a = event_init()  [current_base = a]
b = event_init()  [current_base = b]
...
event_base_free(b) [b == current_base -> current_base = NULL]

So current_base ends up prematurely set to NULL.

Test Plan:
Run dba/core/daemons/dbstatus/dbstatus_tests.lpar, which no longer
segfaults

Reviewed By: jsedgwick@fb.com, davejwatson@fb.com

Subscribers: dihde, evanelias, trunkagent, njormrod, ncoffield, lachlan, folly-diffs@

FB internal diff: D1663654

Tasks: 5545819

Signature: t1:1663654:1415732265:d51c4c4cae99c1ac371460bf18d26d4f917a3c52

Blame Revision: D1585087