Re-enable public constructor for ClientAsyncResponseReader to avoid busting client that bypassed code generator. This code is deprecated-on-arrival as it is a performance pessimization. This code path should not be used.

Fixes compilation if building with -Wextra-semi.

This reverts commit 79759fea, reversing
changes made to dc36f4df.

C++ Compression Level set properly

with a Sneaky member are in a collection.

whenever appropriate so as to avoid any unintentional free-before-use
problems.

Potential performance issue: this triggers an additional allocation
for each Async call initiation, along with the cost of ref-counting
shared_ptr . But this is worth it for the additional safety provided
here without any change to the exposed C++ API.