Closes #2639. Adds optional limit parameter with a default value of 50.

The new parameter is to satisfy the principle of least astonishment
by providing a sensible default for the maximum number of transparent
inputs to shield.  If users do not configure -mempooltxinputlimit
it is possible for them to create transactions with hundreds of
inputs which suffer from mining delay, due to the current state of
the network where some miners have configured -mempooltxinputlimit
as a way to deal with the problem of quadratic hashing.

src/rpcclient.cpp

@@ -110,6 +110,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "z_sendmany", 2},
     { "z_sendmany", 3},
     { "z_shieldcoinbase", 2},
+    { "z_shieldcoinbase", 3},
     { "z_getoperationstatus", 0},
     { "z_getoperationresult", 0},
     { "z_importkey", 2 },