Configure JVM initial heap size

To enable some performance testing, this change will
allow configuration of JVM initial heap size. A new
environment variable will be read when the instance
starts. The variable is named JVM_INIT_HEAP_SIZE_MB.
The unit is megabytes. Setting the variable to any
value above 0 will cause SQL set the initial heap size
for the JVM.

This change does not effect the setting of the JVM
maximum heap size. As before, the maximum is set to
a hard-coded default, currently 512MB, unless the
variable JVM_MAX_HEAP_SIZE_MB is set. Setting this
variable to zero has the effect of not configuring
any maximum.

In contrast, to avoid setting any value for the JVM
initial heap size, one can simply not define the
variable JVM_INIT_HEAP_SIZE.  The reason for this
inconsistency in how to use the variables is to
preserve default behavior.

So, for example, to setup the JVM to have no maximum
heap size, while having an initial heap size of 32MB,
one must set both variables, as shown here:

JVM_INIT_HEAP_SIZE_MB=32
JVM_MAX_HEAP_SIZE_MB=0

Change-Id: I4f503eda59f263a2762adb79b7d79ef158a3852a
Implements: blueprint jvm-inital-heap-config
Merge "Configure JVM initial heap size"