Introducing iomgr.

Move eventmanager and platform dependent endpoint functionality into a single
library called 'iomgr'.

This is primarily to prepare for a Windows port - where posix socket semantics
lead to poor quality code.

Mostly this is a code movement CL, with some small changes to help prepare the
way for porting:

- em style fd objects can only be held internally in iomgr, and own their memory
- added grpc_iomgr_create_endpoint_pair() to accomodate the common pattern of
creating a tcp endpoint from the output of socketpair - this will help keep
our tests portable
- separated em alarm interface into a separate file, as this part of event
manager is needed higher up the stack
- made the eventmanager bits a true singleton, simplifying API's across the
stack as there's no longer a reason to carry a pointer there.

Initial design document is here:
https://docs.google.com/document/d/1VmafcHvvrP5kwtQkz84R5yXF7u7fW-9Pn0bkSUQHDt8/edit?disco=AAAAARNByxg
	Change on 2014/12/09 by ctiller <ctiller@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81716456