Thrift: Revamp TMemoryBuffer constructors.

Thrift: Revamp TMemoryBuffer constructors.

Summary:
There were some weird cases where the implicit conversion from
const char* to std::string was causing the wrong constructor to be called.
There wasn't really a clean workaround, so we're dropping the string
constructors.

Reviewed By: mcslee

Test Plan:
Ran the test.
Grepped around the /projects tree for uses that had to fixed,
and fixed them.

Revert Plan: ok

Other Notes:
This risk was pointed out by Ben Maurer.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665461 13f79535-47bb-0310-9956-ffa450edef68