pgsql: Avoid statically allocating statement cache inecpglib/prepare.c

Avoid statically allocating statement cache in ecpglib/prepare.c.

This removes a megabyte of storage that isn't used at all in ecpglib's
default operating mode --- you have to enable auto-prepare to get any
use out of it.  Seems well worth the trouble to allocate on demand.

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e15aae829e3eb686bdf03dea9022a61f8039a7ef

Modified Files
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src/interfaces/ecpg/ecpglib/prepare.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)