Starting to allow for >1 implementation of pollset within a binary.

Do so without requiring an extra allocation for completion queues (which
we could not tolerate).

Refactor end2end_test. Make the credentials a separate library.

Load balancing documentation first draft

Skeleton Summer of Code ideas page

This reverts commit 5152cd29.

Rename C# Preconditions to GrpcPreconditions

Add options to Node library to modify ProtoBuf behavior

Increase Node's per-test timeout and Ruby's overall test timeout

Fixed refactoring of grpc_arg pointer vtable

Subchannel Sharing  [The interop tests failures and Basic test failures are unrelated to this change. Merging]

Provide explicit API for user to set user agent string prefix

Add an open-loop test using Poisson at 20-70% of peak

Flagging the missing API entries.