Set default TBinaryProtocol.readLength to 10MiB (#1530)
Set default TBinaryProtocol.readLength to 10MiB (#1530)

Thrift request declares size of its fields. TBinaryProtocol allocates
buffer for thrift request fields before reading actual field. Thus
specially crafted request may cause namerd to allocate buffer of size
up to 2^32 bytes.

This commit sets environment variable which limits thrift field size
(so called readLimit) to 10 MiB globally. This provides safe reasonable
default and also suggests setting this variable in case if JVM_OPTIONS
is set.

Change is validated by sending specially crafted request from bug
report. Namerd rejects request.

Fixes #1524
BuoyantIO/linkerd,linkerd/linkerd,linkerd/linkerd,denverwilliams/linkerd,BuoyantIO/linkerd,denverwilliams/linkerd,linkerd/linkerd,denverwilliams/linkerd,denverwilliams/linkerd